If you didn't hold him down, for that purpose, would he be still alive?
If I knew he had a gun and intended to shoot somebody and I went along to try and dissuade that action, no foul.
If I knew in advance that my buddy intended to commit an illegal act and I went along to assist in some way, I'm equally guilty of any offense my buddy commits. My assistance can be quite passive, such as a look-out, driving the getaway car, or anything tending to make a success of my buddy's escapade.
Let's take a simply example: If I hold the victim down while my buddy stabs him 18 times thereby ending the victim's life, should I be responsible only for simple assault? After all, I didn't do any stabbing.