Do the murders actually have to take place, or is an attempt good enough? Mewtwo tries to kill almost everyone on the planet, but Ash and co. stop him with the power of friendship before it can happen. He's only killed a handful of people in the past.

Mewtwo has never lost a battle in canon, a-and I kind of like him, so it would have to be a failed attempt. A really sneaky, skillful failed attempt, possibly followed by >9000 power being blasted at Fox if he wasn't knocked unconscious immediately. It's hard to kill a living weapon!