He's a coward. Rides the whole "messiah" thing to get revenge for his family, thinking he can avoid the bad parts, then it happens anyway. He glimpses the Golden Path but instead of going through with it he gets depressed and dies. If his son didn't do the job for him, billions would have died in his Jihad and then humanity would have gone extinct anyway. I remember Leto II's feelings towards his father were... complex in God Emperor.

Anyway I'm very happy with Villeneuve's adaptation because he did a much better job than Herbert at writing Paul as a Villain (Herbert famously regretted that his depiction wasn't clearer which is the why he wrote Messiah).