Marluxia is evil. There's no doubt concerning that. He isn't called "assassin" for no reason, he really likes to kill, and his weapon is a giant scythe. Most of the people don't like him, and laugh at him because his hair is pink and his scythe releases pink flower petals whenever he swings it. But it has a special meaning: In Japan, the color pink represents death, so he is a death agent. The petals means the death coming. His scythe can be associated with a death scythe, and there you have a Shinigami, a god of death. I really like this hidden meaning, 'cause it makes him even more awesome, and explains his attitude and his title.
He was given leadership over castle oblivion, and was also the head of the conspiracy group that planned to take over the Organization. Along with Axel, Larxene, Zexion, Vexen and some others, he was eliminated by Sora (Sora, Riku and Axel killed all of those there, except for Axel himself). He's the final boss in Sora's part in CoM. He's the one who had the idea of using Naminé to control Sora.
As every good boss, he has more than one form, to complicate our lives. He has four Badass forms, and is hard to beat.
The three last pictures are Marluxia's Second, third and final form. the first is his usual form. the hardest one to beat was the third, the angelical form.