This thread is for posting examples of fictional characters who've gone mad with power.
To start off: the two teenaged vampires from Hellsing (The manga, the first anime, and the second anime) who murdered at least one family in the first anime, and at least two in the manga and second anime). In the first anime they went off about how they either were going to or wanted to get stronger ( didn't get what was going on in their heads, I didn't see how they were going to pull it off.)