Chances are they went ballistic because the driver left their friend to die, most people who ride know that accidents are a thing, and when that happens it won't be pretty for you. You're assuming that everyone who rides a bike is a "biker tough guy", but even if that's the case, what are your options?
If you try to drive away, they're going to get to you the moment you encounter some form of traffic, and if you're lucky enough to avoid that, you still are going to ram them off the road to lose them, and I doubt that you'll get to claim "self defense" when the cops come knocking at your door.
Meanwhile, if you stop, get out, and at least attempt to fake some empathy or care about the man who's life just ended, you might avoid a beating, and no one can charge with hit and run. Also, you get bonus points for acting like a decent human being, and not some coward who flees from trouble.