It was Raph's fault that Splinter was captured in the first place. When he rescued April from the subway ambush by the foot clan, he led them right back to their hideout. He should have been a better ninja, and made sure he was not followed. He looked back one time while running away, but wasn't thorough enough. It could also be argued that he was responsible for April's antique shop being burned to the ground, since that fight would've most likely been under control if the 4 brothers were together.

Splinter warned him to control his anger better. It nearly cost him his life, and the lives of his new friends, Splinter and the other 3 turtles.