Newton's second law (motion)

Any mass that is gained or lost by the system will cause a change in momentum that is not the result of an external force.

question: how do you know when enough is enough? does having a bigger perspective of this world and the situation you're in guide you to a wiser decision? or would all this information become too much, rendering you unable to really partake in the process to either cease or continue in an unaltered manner?
when you see a man abusing a woman, how do you know how to react? you're not scared, you're not in any position that could get you injured in the process, so why not just do it, and stop the abuse?
yesterday i had a similar encounter and i couldn't help but notice how quickly i found myself losing interest in the situation. i could have prevented something from occurring but instead i let it take its course. does that make me a bad person? does that make me responsible for what happened? by doing nothing, was i doing something? if i wasn't doing anything then why do i feel guilt? agh trial and error next time i'll be wiser to do otherwise. :/