From Obi-Wan's POV he was seduced by the dark side, but he really was "seduced" by its potential to save his wife and kid. I see no contradictions. It was just, again, different from what a lot of people expected.
Well, I can see that but there is still a gap of how he suddenly is a crazy murderer? Wouldn't it be more in his character to say kidnap the emperor, try to beat his secret life elixir out of him and run away with Padmé?