Trouble in Scarlet
After realizing that her magically-induced twins had been lost and the very memory of them had been hidden from hero, the Scarlet Witch found herself in a bad mental space, partially due to her dealing with a cosmic demon. In AVENGERS #500-503, she used her massive magic abilities to send everything from Ultron to the Kree after her one-time teammates, seriously injuring She-Hulk in the process and apparently killing Jack of Hearts, Scott Lang AKA Ant-Man, and Hawkeye.
Had Doctor Strange, Captain America, and Magneto not intervened, there's no telling how much death and destruction Wanda Maximoff could have wrought in that damaged state. Shortly after, she completely rewrote reality resulting in the HOUSE OF M event. When things got back to normal, Wanda made one more unexpected choice when she decimated the mutant population with three simple words: “No more mutants.”