A boss

SORRY I RANDOMLY JUST DISAPPEARED I WAS BUSY WITH SCHOOL ALSO IM REALLY REALLY SICK AnYwAy *ahem* this is a boss from undertail i actually don't know nor have played this game so...please dont attack me in the comments if the game's wrong or smt like that anyway this is a boss you have lowered their gaurd you cna suddenly betray them and kill them with a one shot attack or spare them which will you choose? IT'S UP TO YOU