But what about all the killing in the Old Testament?

That is a complaint aimed at Christians who bring violent aspects of Islam to our attention. That complaint is uninformed. The timing of when the various chapters of each set of scriptures, the Qur'an and the Bible, were written is critical. (See "abrogation" and "Moral equivalency")

The basic difference is this:

The Qur'an: As Mohommad wrote the Qur'an, his writings evolved from being peaceful and benign, to later becoming intolerant and encouraging warfare. In Islamic teaching, the latter writings supercede, displace or "abrogate" the prior writings. The "intolerant, warring, supremacist" verses that were written later in Mohammad's life supercede or displace (abrogate) his earlier, peaceful writings. So we end up with predominantly intolerant and violent teachings. Pointing out the peaceful passage of the Qur'an as the basis for insisting that those passages represent Islam is a trick or deception Muslim defenders such as Karen Armstrong and many others play on the uninformed.

The Bible: The Bible's earliest writings, as in the Qur'an, are superceded by the later writings. The Old Testament is superceded by the New Testament. But the big differece is this: The order of peace and violence are reversed. While the Old Testament has many violent and warring chapters, the New Testment displaces many of the violent, warlike writings included in the Old Testament with the prevailing themes of mercy, forgiveness, love, patience and tolerance. The Bible leaves the reader with a non-violent religion.

Even from both a Christian and a Jewish perspective of the Old Testament, as Robert Spencer reminds us, "the consensus view among Jews and Christians for many centuries is that unless you happen to be a Hittite, Girgashite, Amorite, Canaanite, Perizzite, Hivite, or Jebusite, these [Old Testament] Biblical passages simply do not apply to you. "

Beware of our natural tendency to compare Islam to any other religion. Don't. It is unlike any other. It is a total way of life. Not "just" a religion. Take a look at this commentary to see what we're talking about.

Here is more on the dual, schizophrenic nature of the Qur'an...

Muhammad’s Ministry in Mecca (earlier writing in the Qur'an)

Generally Muslims have recognized that Muhammad began his ministry among his own people (the Arabs) in and around Mecca in A.D. 610. For thirteen years he faithfully proclaimed that God alone is God. Yet his followers were few and mostly of lower status. With Muhammad they endured opposition, ridicule and even persecution. Still, throughout this period he responded with restraint. In fact, the Qur’an itself documents how he was to respond to rejection and abuse. The following are a few examples:

I (Muhammad) am but a plain warner. (67:2; passim)^[3]

We have not sent thee (Muhammad) as a warden over them. (17:54)

So proclaim that which thou art commanded, and withdraw from the idolaters. (15:94; cf. 15:94-99)

And bear with patience what they utter, and part from them with a fair leave taking. (73:10)

... And to be of those who believe and exhort one another to perseverance and exhort one another to pity! (90:17)

Call unto the way of the Lord... and reason with them in a better way.... Grieve not for them and be not in distress because of that which they devise. (16:125-127)

Repel evil with that which is better. (23:96)

All of the above passages are Meccan passages, i.e., passages Muhammad received while he proclaimed Islam in Mecca. As long as he remained in Mecca, he responded to his enemies peacefully and with restraint. He never responded militantly. What a fine resource of Quranic references to demonstrate that Islam is peaceful, non-violent!

But then things changed...

Muhammad’s Ministry in Medina (later writings in the Qur'an)

In A.D. 622 Muhammad moved from his home in Mecca to Medina, where Arab tribes had invited him to reside and where they became members of the new Islamic movement. So important is this event in Islamic history – it is called the hijrah ("emigration") – that it actually marks the beginning of the Islamic era.^[4]

In Medina Muhammad quickly assumed both religious and political leadership over the whole Medinan community. Soon after he arrived in Medina, he received the first of many Quranic passages (called Medinan passages) which directed him and the Muslim community to fight in the cause of Allah against their enemies. The Qur’an alludes to Muhammad’s conflicts with the Arab polytheists throughout Arabia, with the People of the Book (Jews and Christians) and with the hypocrites (Arab polytheists who feigned conversion to Islam: cf. 49:14). The earliest extant Muslim biographies of Muhammad detail Muhammad’s military struggles.^[5] It is these conflicts which serve as the seeds for the traditional Islamic divisions of society into 1. the House of Islam and the House of War and 2. the Muslim Community; the People of the Book (Jews and Christians, cf. 9:29,30); the Polytheists (who could become Muslim or accept death or slavery).

The following are a few of the Medinan passages which refer to jihad as military struggle in the Qur’an:

Sanction is given unto those who fight because they have been wronged... (22:39; cf. 22:39-41)^[6]

The (true) believers are those only who believe in Allah and His messenger and afterward doubt not, but strive with their wealth and their lives for the cause of Allah. Such are the sincere. (49:15; 22:78; 25:52)

Fight in the way of Allah against those who fight against you, but begin not hostilities. Lo! Allah loveth not aggressors.

And slay them wherever ye find them, and drive them out of the places whence they drove you out, for persecution is worse than slaughter. And fight not with them at the Inviolable Place of Worship until they first attack you there, but if they attack you (there) then slay them. Such is the reward of disbelievers.

But if they desist, then lo! Allah is Forgiving, Merciful.

And fight them until persecution is no more, and religion is for Allah. But if they desist, then let there be no hostility except against wrongdoers.

The forbidden month for the forbidden month, and forbidden things in retaliation. And one who attacketh you, attack him in like manner as he attacked you. Observe your duty to Allah, and know that Allah is with those who ward off (evil).

Spend your wealth for the cause of Allah.... (2:190-195; cf. 2:216-218; 2:244; 8:38-40; 8:65,66; 4:84; 5:33-35; 61:4)

Then, when the sacred months have passed, slay the idolaters wherever ye find them, and take them (captive), and besiege them, and prepare for them each ambush. But if they repent and establish worship and pay the poor-due, then leave their way free.... (9:5)

Fight against such of those who have been given the Scripture as believe not in Allah nor the Last Day, and forbid not that which Allah hath forbidden by His messenger, and follow not the religion of truth, until they pay the tribute readily, being brought low.

And the Jews say: Ezra is the son of Allah, and the Christians say: The Messiah is the son of Allah. That is their saying with their mouths. They imitate the saying of those who disbelieved of old. Allah (Himself) fighteth against them. How perverse are they! (9:29,30)

O Prophet! Strive against the disbelievers and the hypocrites! Be harsh with them. Their ultimate abode is hell, a hapless journey’s end. (9:73)

O ye who believe! Fight those of the disbelievers who are near to you, and let them find harshness in you, and know that Allah is with those who keep their duty (unto Him). (9:123)

As for Jihad’s motivation and benefits:

O ye who believe! Shall I show you a commerce that will save you from a painful doom?

You should believe in Allah and His messenger, and should strive for the cause of Allah with your wealth and your lives. That is better for you, if ye did but know.

He will forgive you your sins and bring you into Gardens underneath which rivers flow, and pleasant dwellings in Gardens of Eden. That is the supreme triumph.

And (He will give you) another blessing which ye love: help from Allah and present victory. Give good tidings (O Muhammad) to believers. (61:10-13; cf. 9:19-22; 9:111; 2:154; 2:243-245; 47:4-6; 3:195)

... Allah hath granted a grade higher to those who strive and fight with their goods and persons than those who sit (at home). (4:95)

From the above Meccan and Medinan verses Muslims have understood that there is a chronological progression in Muhammad’s ministry from peaceful proclamation only in Mecca to peaceful proclamation supported, if needed, by the sword in Medina. This was not to suggest that God had changed His mind and that peaceful proclamation of Islam had ceased. It simply meant that when Muhammad entered Medina, initially he was allowed to defend himself against his enemies with the same weapons they used to attack him and eventually was ordered even to fight all idolaters.