Magneto- X-Men: Ultimatum
In X-Men: Ultimatum Magneto's ultimate goal is to destroy the world due to the death of Scarlet Witch and Quicksilver in Ultimates #3. He gains possession of Thor's Hammer and uses it to amplify his powers and cause natural disasters in an attempt destroy the world. A tsunami hits Manhattan and a lightning storm threatens New York City- killing millions of innocent people and a number of "ultimates" like Valkyrie and Captain America as well.
Professor Xavier reveals Magneto to be in a floating citadel with Thor's Hammer but all of the Marvel heroes are too busy mourning their dead to intervene. Magneto makes his way to Xavier's Institute for Gifted Children where he reveals his plans to Charles before killing him. In killing Charles Xavier, Magneto solidifies himself as the most villainous adversary the X-Men have ever faced.
The events of Ultimatum are significant because they further prove that Magneto has no regard for human or mutant life and, in the end, he is either hellbent on revenge, control, power, or all three.