You forgot a 'h' there. It's Sephiroth, not 'Sephirot'. :)
(I am kidding, don't hurt me!)
Anyway! It's possible that Thancred recalls a lot of stuff from his time being possessed by an Ascian - so he may feel as though teaming up with them to save Minfilia is a 'wise' move on his part. Desperate people do desperate things after all - and he's prone to punishing himself severely/not talking out his problems and acting recklessly instead.
