Sometimes, you just gotta shoot Bambi
To begin with this, let's set the scene. I prefer a timed game rather than stock for 3-4 players so that all players are involved the whole way through. Nobody ends up waiting on other people to finish except for the standard "you lost, give up the controller to the next player."
Now, within those settings for a match, take the most basic example. I'm playing my best character, Donkey Kong (don't laugh, I'm better than you'd think). One of my opponents is playing Snake, and pretty good at doing so. Another opponent is playing Yoshi, and not all that well even for Yoshi. Who do you suppose is my target at the start?
Initially, that'd be Snake. All else being relatively equal, Snake is the bigger threat. He's a good character and the person playing him is a skilled opponent. I'm in far more danger of him running away with the game than Yoshi. Yoshi I can write off as not winning this fight, but hopefully the player is still having fun with the game at hand (I'm pretty good at setting things to work that way). If I can get Snake KO'd, I give myself a point and take away a point from the best opponent. This clearly works to my advantage.
Now let's say the match has gone on a bit. Snake and I have been rough against each other, but neither of us has really been able to make anything stick. Yoshi, bless his heart, has been trying to get in on this and predictably failing. He decides to retreat and let us damage each other while he goes hunting for items that might even things out for him. Just for a random high number, let's say he's gotten to 139% damage, while Snake is sitting at less than 50% (mine is mostly irrelevant for this discussion). I don't mean anything against you Yoshi, but there's a big bullseye right on your back. I can always resume my fight with Snake, but even if I absolutely never get any kills off of him, I still win if I have the most points. Right now, Yoshi is a source for one of those points.
Option #3: I'm off in the middle of nowhere while all of my other opponents are in one big brawl together. The dojo of not attacking is a powerful one indeed. Why should I fight if I don't have to? Conflict is messy, I could get hurt. The end result wouldn't be much different if I got involved, except I'd come away with some bruises. After blows have been traded pretty well, eventually someone gets tossed over in my direction. If I don't outright kill him, I can at least land a good strike or combo that makes it easier for me to do so. The victim in this case can easily feel like he's getting picked on, but all that's really happened is that I caught the bone that was thrown to me. Should I simply not attack when an enemy is thrust upon me? I don't see why.
Let's get a little more specific here. There's 4 players. Again, I'm playing against Snake. Yoshi's still here, but now we also have Jigglypuff of moderate skill. I'm still hanging out at the other side of the screen while the other 3 are brawling together. Let's say Snake has just been cleaning up and gotten both of the others mighty hurt. Do I go in there and try to get those kills? Probably. I don't really want him nabbing extra points, because his skill is similar to my own. I'll take a risk of getting hurt, but I really want to be the one taking those points instead of him. What if Snake's the one getting hurt while Jigglypuff and Yoshi are pretty low in damage? I think I just might stay out of it. I don't care so much if either one of those guys gets the kill, but once I get involved, I'm adding to the distraction. Snake now has a higher chance of getting out of this right into the position I previously had, and I might be the one getting beat. In this case, it's probably best if I close in, but still let them do their thing. I'll provide backup if I see Snake starting to escape.
In this case, yes I do become a bully, and make no apologies for it. I have no choice but to recognize the lower players for just what they are: cattle. The rest of the match could've been just the two best players trying to get kills off of the dummy players, but we had more to gain by targeting each other. Not so when the round is about to end. Of course, in my racing to get these kills, I end up injuring myself because I'm going to meet resistance. Snake is well within his rights to capitalize on this and try to get a big advantage by killing me. All I can do at that point is take advantage of the aggressive posture, make myself appear weak to him, and hope I can pull off some shenanigans. Make no mistake, I am desperate and embracing zero mercy to eke out a win.