No I don't think it was Clouds fault. Sephiroth/Jenova was clearly the one who physically killed her. She was the one who decided to secretly go alone down to the alter. Appearently it would be sooooo wrong for her to bring a bunch of strong friends to be there during an important moment. As Jenova (the user) said Cloud resisted killing her himselfso under the circumstances he did better then nothing. I can understand how it wieghs on him cause he couldn't/didn't do more but it would of been a different story if Aeris actually just made it a team effort (someone could of tackled he out of the way/deflected the blade/tried to warn her)