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Usually this attempt fails and becomes a Backstab Backfire. Pratt is an Ungrateful Bastard.
Edited by eroockSurprised something like Mercy Backfire doesn't exist. In this case, if you consider Hillel refusing to shoot Pratt a form of "pacifism", it could count as Pacifism Backfire. Also verges on Violence Really Is the Answer - if Hillel had just killed Pratt, it would have saved "the sweetheart".
Edit: Maybe an Inversion of Mercy Rewarded too?
Edited by BeerBaronCould qualify as Too Dumb to Live, especially since even if Hillel didn't want to kill, at least they could have checked Pratt for weapons, put them in handcuffs, hand them over to authorities, etc.
But just saying "Now that I've proven I'm better than you in a fight (or whatever), I'm just going to immediately turn my back" is pretty dumb.
Edited by KDIf Pratt admitted defeat in whatever fight they were having, it's I Surrender, Suckers!. If not, and Hillel was simply refusing to kill him out of principle, it's Pacifism Backfire.

Hillel refused to shoot defeated Pratt. Spared Pratt, diabolically smirking and gigging, is pulling out a gun and is about to shoot the hero.
"Well done, you gave a chance >:D "