Lady Stonegate

...

She used to be a guard at the Stonegate Penitentiary, the old prison under control of A.E.G.I.S., build to confine villains. After years of a near perfect record, one night she just suddenly attacked all the other guards, using powers no one knew she had. She managed to free only one of the villains, all the others she either killed or badly wounded. She left none of the A.E.G.I.S.-Agents alive, used her powers to crush their heads or took all their blood.

Even after years of researching her and going over all her files, the officials still have no clue why she acted like that out of the blue.