You may not understand, but sometimes its not as easy as just "reporting" it.
Im sure he wouldn't have purposely hopped in the car if he knew they were on their way to go murder someone. Just because you're in the car when something suddenly goes down, you're not responsible for the actions of 2 other grown men.
Now back to the reporting it thing. Im not sure where you're from, but where im from, "reporting" something can get you killed or your family members hit up quickly!!!
I don't know how many people would consider "reporting" the right thing to do, knowing you've just jeopardized the safety of you and yours. So honestly, i think he did the RIGHT thing by not initially reporting anything.