>But you can see in your example if one of the teenagers holds the man down and another one stabs him that maybe they both killed him, even if only one of them used the knife?

Of course, that would be understandable. And I don't think that would need the "joint enterprise" construct to be tried as both being murderers in most legal systems.

The unjust concern is for the people who weren't such active participants in the act of murder (or called for it), but just on the "initial" crime.