Batsy isn't the only one who has witnessed, and done, some dark deeds in the DC Universe, because the Flash has quickly become one seriously dark dude!
(If you watch the CW show then you probably know some of these storylines already, but if not be warned there are spoilers ahead.)
Barry Allen has had his psyche damaged by a past he cannot escape (the death of his mother) and the earth shattering results of his actions (Flashpoint) but nobody got in to Barry's head like his nemesis Zoom.
Zoom made it his mission in life to take down the Flash no matter the cost, and as far as super villains without a conscience go Zoom ranks up there with the likes of Darkseid and the Joker in terms of body count.
But one kill in particular caused Barry to forego his heroic ethics and kill Zoom with impunity- the death of his wife Iris West.
Zoom murdered Iris because he was in love with her too, and because he had a psychotic infatuation with watching Barry suffer, but despite his anger the Flash let Zoom live- so Zoom showed up a few years later to kill Barry's new fiance on their wedding day.
Barry couldn't bear to see another woman he loved get killed by Zoom, so he took matters into his own hands- by snapping Zoom's neck before he could deliver the killing blow. It was a justifiable homicide, but it still took its toll on Barry's state of mind...