Maybe Stryfe because he wants to exact revenge on his biological parents just for not being there in his childhood? And also he wants everyone to feel his pain? Could be seen as immature.
Perhaps Stryfe's arrogance can be seen as immature? "I am the most powerful mutant on the planet ... past, present and future."
Xavier does not seem immature to me.
No spite intended. Please keep the conversation polite and proper. Thanks. : )