The man who would become Meme was hiding in the Morlock tunnels and was not aggressive at first. That was until he started to use his ability to absorb humans into himself, provoking the rage Jean Grey and Colossus. During the fight Meme explains how his powers work and Colossus was able to take him down. However with no other option, Jean reached into his mind and turned it off instantly killing him. An action Colossus saw as too rash.
Meme may not look like it but he was quite powerful. His ability to absorb others into himself in order to create extra mass and appendages make him complicated to fight, even more so if you were unwilling to kill those that were melded into him. It was not known if Meme also accessed the knowledge of those he had absorbed and if he absorbed super-power individuals would he have gained their powers.