This is a character who wants to destroy everything and everyone in the story, and is suddenly in a position to do so (on a small or large scale); but in contrast to the Ax Crazy Omnicidal Maniac, he/she has some plausible, outright tragic reason to do so, so much so that, in some cases, the characters outright sympathize with him/her (whether or not the audience does too is out of the equation here), all the while accepting that he/she must be stopped. Sometimes, the character is even portrayed as likeable, just...not with the right mindset.