Et Tu, Brute?  Patricide?

(Actually, in Plutarch, the assumption that Brutus was Caesar's natural child is made quite explicit, though certainly not considered absolute truth.  Plutarch states that it was believed by everyone (many people) that Brutus was in fact his son, and Brutus' mother and Caesar did have an intimate relationship around the time Brutus was conceived.  Many people speculate that this is questionable, but in fact, the logic of it makes good sense: Caesar would have been about 15 years at the time, he was having or did have an affair with Brutus' mother, Servilla, who was also Cato's sister, and the time of the affair was around the time of Brutus' conception and birth.  Whatever the case may be, it appears Caesar believed it to be true, others also thought it true, the entire senate knew of the affair - as it was made public due to Cato's actions in a senate meeting in the debate when Caesar received a note (Cato [& Cicero] called out Ceasar for allegedly corresponding with the enemies of the commonwealth; it turned out that in fact, the note was a love letter from Cato's sister!).  Therefore, whether true or not, it was widely believed to be TRUE - by Caesar and many others, which may have included Brutus, too.  Caesar seems to have viewed Brutus as either a son or a family member.