Matt, I learned my first lesson about getting involved back in 1974, and got slashed for it...resulting in having a bit of finger grafted back on.
In 1977 or 1978, I was about 25 at the time, I was walking down the street with two gals from my office to go to an after work "bowling league" thing we did for fun once a week. As we went down the street, we heard and then saw a guy beating a woman and whipping her with his belt.
I have never tolerated bullies or people preying on the defenseless well, ran down the street and yelled at the guy to fight me if he wanted to fight someone. He stopped, looked at me, (not a big guy at 5'10" and maybe 170 lbs back then), and started making excuses that he didn't hurt her as I pulled the belt from his hand and pushed him back against the car he was beating her against.
A few moments later, the "victim" told me to "mind my own business" and that it was "a boyfriend girlfriend thing".
I told her that they deserved each other and walked off. The two gals from the office dubbed themselves "Charlie's Angels" after that...and I decided if I was going to continue to be stupid enough to interfere in such circumstances, I'd better get training...and joined a dojo.
(It was in fact a year or two later that a girlfriend of mine was attacked, beaten and robbed on the street in Brooklyn, (leading to hospitalization), that led to my pursuing a career in law enforcement, which had never been prominent on my radar prior to that incident.)
All that said, I've been lucky to not have been (badly) injured in any of the scenarios I inserted myself into...but now, at 70, I'm pretty much done with it unless someone's facing grievous bodily harm or death.
Edit to add: The one vow I've made to myself is to never pass an LEO on the street in need of assistance from an assailant. That's a promise I have lived with for over 40 years and don't intend to break as long as I am able.