I feel like Cloud had a natural reaction to the betrayal of his role model and idol. The important question to ask is if he still felt like Sephiroth was previously a role model to him later on in the story and during the course of the game.
you mean after the whole getting dropped into the lifestream thing? the final battle wouldn't make much sense if he still wasn't. Cloud was only able to defeat his idea of Sephiroth at the very end.
This twenty-year-old boy was distinguished from childhood by strange qualities, a dreamer and an eccentric. A girl fell in love with him, and he went and sold her to a brothel...