Originally posted by Anakin4Ever
Anakin is pretty bipolar. Going from a slave to a Jedi was a huge step for him, so going from a Jedi to Sith may not have seemed so implausible. I do agree it's kind of weird how quickly he turned.
He did it for his wife and baby, though.
Same reason Jacen Solo became Caedus, though at one point he was duped into believing he had to kill them to become a true Sith.