Mephiles550

I'm surprised no one mentioned Anakin (I'm an Anakin fan, but even I can't deny that he made some bad decisions, even though in the end he fulfilled the Prophecy).

Definitely Anakin, he killed all the Jedi in the Temple including the younglings because he didn't like sand or people with the high ground. "In my point of view the jedi are evil" after killing 8 year olds. 

I'm going with Mace Windu on this one.

Basically said the dumbest shit to convince Ahsoka to stay, which ended up making her leave anyway and he probably didn't care.

Brought only three other Jedi Masters, one of which Grievous even technically bested,  to take down Palpatine without properly examining the situation.

Made his own padawan fall to the Dark Side and he had to kill him.

Treated Anakin worse than any other Jedi did IMO, first one to declare that he should not be trained, did not believe that he was the chosen one,  kept denying him important misions and trust, etc. Always failed to properly reason with Anakin.

Fell for Palpatine's bait, both before confronting him and after "beating" him right as Anakin showed up.

Basically I'm convinced Windu is the whole reason why all the fucking Jedi were killed cause of his poor decision making.