Obi-Wan's mistake was training Anakin in the first place. Remember he was against Anakin being trained at all because he knew how dangerous the boy was.
"The boy is dangerous. They all sense it, why can't you?"
Obi-Wan only trained Anakin after Qui-Gon told him to, but that doesn't necessarily mean that he agreed with it. He just loved Qui-Gon so much that he was willing to do whatever he told him. Maybe it was this reluctance to train Anakin that led to the bad training methods (holding back emotions, scolding, and everything else that people have said).
And by the way, it is ridiculous to me that one would get promoted to Master just by killing a Sith. Obi-Wan wasn't promoted to Master after Episode I when he killed Darth Maul. No, you get the rank of Master when the council feels you deserve it.