If the plan was "go into this house and kill the occupants" and the group executed that plan even though only one person pulled the trigger, that would be one thing.

The plan was never "go into this house, wait for the cops to show up, get into a shootout with the cops, and get shot by the cops". Or, if it was, the kid who was charged sure didn't follow that plan because he ran away instead or getting into a shootout. There are a variety of possible crimes for someone who was part of a group: conspiracy to X, reckless endangerment, negligence, etc.

At a bare minimum, if someone in a group is charged with X, it should be necessary to prove that the group's plan was to do X. In addition, it should be necessary to prove that the group did X. That seems like it should be the absolute minimum. In this case, what's absurd is that the group didn't even do X.

In this case, the "murder" was the "murder" of one of the criminals, and the person who did the "murder" was the cop. It's absolutely ridiculous that if the cop were charged with that murder (and somehow wasn't just automatically immune) the cop could invoke their right to self-defense, and would almost certainly be acquitted. But, the person who was running away from the crime scene at the moment the murder happened can't use the self-defense justification because he wasn't the person who fired the shot.