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I've definitely seen this a lot in the sense of having a hero or hero team who are strong fighters, so it's not a big deal for them to intervene in a fight and say that regardless of the circumstances, outnumbering and ambushing someone is never okay. It's kind of a well-hedged bet. An innocent should be protected, and even a bad guy deserves to be tried in a court of law, not be lynched in the street.
The closest thing I can find is Chronic Hero Syndrome (which is more about helping everyone in need, not just evening up an unfair fight), and maybe The Dulcinea Effect in some cases (if the outnumbered side includes a beautiful woman that the hero will now fight an army for). Unless I'm missing something, it seems like there's a gap, and I could readily give a few other examples.
Edited by KD
Chronic Hero Syndrome if it's about rushing to the rescue because it's just the right thing to do, but maybe Let's Fight Like Gentlemen if it's more about not liking an unfair fight than straightforward altruism per se (Fair-Play Villain and Villainous Rescue if the character is an established villain)? What they're rescuing them from, the number of Mooks or size of the Gang of Bullies in the fight, any Conservation of Ninjutsu in effect and the One-Man Army status of the rescuer, I think those are separate from the latter's actions and motives, if you see what I mean. Might also want to see Innocent Bystander and Damsel in Distress and their related tropes for the person being rescued. Bully Hunter and Vigilante Man may also apply.
Edited by Unsung
Is there a trope for the story beat where a heroic character sees someone outnumbered in a fight and jumps in to even the odds, even though they don't know the context? This is of course very common in the classic Arthurian stories and in those types of heroic fiction that share the same ethos.
I'm thinking of things like the meeting of Lancelot and Arthur in the 50s movie Knights of the Round Table, where Arthur sees Lancelot beset by an ambush and jumps in on his side, knowing nothing about who is who, or in a similar vein, the Babylon 5 episode "A Late Delivery from Avalon," where G'Kar sees the ersatz Arthur outnumbered by thugs and jumps in to aid him, despite not knowing the cause of the fight.