Kevin MacTaggert had his reality warping and possession powers, but he also had the ability to change himself back into his human form as well, a power he did not have in the comics.
Also, when he took over people's bodies, it did not kill them like it did in the comics, it only weakened them. Proteus himself was somewhat humanized and much less evil; he was a violent, unstable, 17-year-old adolescent with a child-like mentality and little grasp on reality (Moira kept him practically locked in her center for years), and at a certain moment he even saved a young man from being beaten up by a gang of bullies.
Kevin/Proteus took off in search for his father; not knowing the truth and craving for his father's love, he blamed his mother Moira for not giving his father a chance. He then attacked Professor X, Rogue, Moira, Wolverine, and Beast, defeating them and making Wolverine experience death, which causes Logan a great deal of psychological damage.
Moira tells Professor X that Proteus' father, Joseph, did not want him because he was a mutant. Professor X and the X-Men confront Joseph himself, but he refuses to listen and is only bothered about people finding out he is Kevin's father because he is a mutant. Rogue is set to protect Joseph, and she hears his speech about loving children, thus triggering her to remember her past of when her father rejected her due to her own status as a mutant. Kevin arrives at the hall and attacks Rogue and Beast as Joseph leaves.
Professor X tries to reason with Proteus and he blames the Professor for keeping Joseph away from him. Wolverine arrives in time to save Professor X and Beast. Professor X tries to contact Proteus mentally and tells him he can help him, but Proteus insists on seeing his father. Moira says they've tried to reason with him enough and tells them she needs to do something about it now. Kevin arrives at another speech of Joseph's, saying that he wants Joseph to love him but Joseph says that he's a trick and trying to ruin him.
Joseph tells him to leave and Kevin gets so mad that he loses control. Professor X manages to stop him by using his psychic powers, calming Kevin down; he returns to his normal shape and gets a hug from Moira. Joseph arrives, apologizes to Kevin, and the two reconcile.