Stalker?

Before they left for the trip, they knew they were going to see him. James and Whitney had felt his presence all throughout the week, but their fears were confirmed when they saw a glimpse of him through the window last night. They decided they would go pick up their son Garrett who lived alone in California after leaving home six months before. The couple, now empty nesters, felt alone without their son until last week when it started to feel like someone was watching them. That's when it all started to go wrong. They decided they would go to California to pick him up, and then go to a nice beach house on the ocean to calm down and visit. They quietly hopped in the car in hopes of keeping whatever was out there at bay. James pulled out a small handgun and placed it in his lap. 

“Just in case,” he told his wife, which brought the tension level in the car up a couple of notches. They started out on the driveway, when all of the sudden a figure jumped out in front of the moving vehicle. James and Whitney were thrown forward as the brakes screeched and a gunshot went off. After the screaming was over, they took a couple of deep breaths and stepped out of the vehicle to inspect the damage. The second they confirmed that a body was on the asphalt, Whitney pulled out her phone and dialed 911. As she did, they stepped closer to the crumpled figure on the ground. Finally they could start to make out features, and there lying on the ground in front of the car was the all too familiar form of Garrett. Lying next to him was a large poster with the words “Happy Anniversary” written on it. The phone dropped from Whitney's hands as she rushed over and turned the body over to confirm that it was in fact their son, Garrett. All that aggressive knocking and feeling of being watched was just Garrett trying to reach them. That's why he hadn't been picking up their calls. The surprise party walked out from behind the brush. Looks of shock and horror filled their faces as they looked at the scene in front of them. James and Whitney turned back to their son lying on the ground and their hearts stopped. What had they just done?