As everyone who has ever taken on the mantle of the Flash is a pretty peaceful soul, you'd expect that the character's superhero career has been without any kind of cruel or awful deeds. Bright, cheerful, and always with a witty quip, it often seems as though the scarlet speedster is DC's break away from their darker, edgier characters.
While this is a totally reasonable assumption, it's also a wrong one. Between the communal struggle to control the Speed Force, and Barry Allen and Wally West's aggressive desire for vengeance, the history of the Flash is littered with a series of concerning events, ranging from relationship issues to downright torture.
Their intentions are always good - it's just that these good intentions don't really excuse countless murders and world-ending events.
This isn't to suggest that the Flash is a monster, but rather that we often only see glimpses of his darker side, making them somewhat easier to forget. At the end of the day, the hero is still largely one of the most well-meaning and friendly members of both the Justice League and the DC universe... just so long as you don't get on his bad side.