Even before he knew Padme was dead he had already become inundated by the dark side. He had, already, killed so many people at the jedi temple but most importantlymore revealing was his conversation with Obi-wan, before their bout, was most telling. There, he tried to impress upon Obi-wan the jedi's impudenceimpotence and how he would bring piece to the galaxy and to his 'new empire'. This represents 4 very influential ideas that each, by themselves, could, understandably, change a person.
- He murdered many people and felt he could not go-back from the path he had chosen
- The jedi ideals were inherently flawed and a new form of thinking was necessary
- He truly desired peace and felt that unilateral rule was the only effective vehicle to accomplish this goal (as many tyrants in the real world have believed.)
- He became power hungry- he was a very powerful jedi and as a wise man once said, "absolute power corrupts, absolutely." He believed all the hype of him being the chosen one and felt that his dominance was required for a better galaxy.
All but the 1st point were motivated by his desire to do what was right (in his own misguided way).