From a narrow legal standpoint, I don’t see why you couldn’t do that — they do it in the other direction already for felony murder. The driver of the getaway car’s intent to particpate in the felony supplies his mens rea for the murder that happened while he was in the car.

This would just be move the intent from the rescuer to the murderer rather than from the murderer to his co-felon.

But can’t have people thinking they’re pulling the “lever” for “the “greater good.” Just something to bring up in self defense after you act out of instinct. Sorry if I went a little afield of your initial post — I just despise the uliltarian mentality so much, and the trolley problem is the quintessential example of it.