The problem with the NAP is that "aggression" is not particularly well-defined. In many cases, a sin of omission (refusing to do something good/necessary) can be just as damaging as a sin of commission (doing something bad). If I find someone in cardiac arrest lying on the sidewalk, I may be "aggressing" them (even breaking their ribs) by performing CPR, but it's probably the right thing to do. Of course, if that person desperately wished to die, my CPR would be far less welcome. "Aggression" is subjective too. Plus: the NAP isn't close to useful when it comes to answering textbook moral cases like the Trolley Problem, or the problem of moral luck.
It's often referred to as "the initiation of the use of force", which covers intimidation/coercion and physical violence of course. That's clear enough. Everyone knows when they're being coerced. Obviously your intent matters too. Genuinely trying to help someone can't sanely be considered immoral. So what? It can be applied to 100% of what happens in ordinary, everyday life. What does a theoretical scenario like the Trolley Problem matter with regard to real life and the kind of situations you actually encounter?The problem with the NAP is that "aggression" is not particularly well-defined.
If I find someone in cardiac arrest lying on the sidewalk, I may be "aggressing" them (even breaking their ribs) by performing CPR
the NAP isn't close to useful when it comes to answering textbook moral cases like the Trolley Problem