OBI-WAN: (to Qui-Gon) The boy is dangerous...they all sense it. Why can't you?
Seeing Obi-Wan and Anakin spar, I can't help but think that Obi-Wan either failed to see a lot of warning signs or--and perhaps more likely--he believed that he could teach and mold Anakin into a proper Jedi.
The latter. He was a good man who so wanted to honor Qui-Gon's dying request that he overlooked or downplayed certain behavioral indicators and wanted to see the best in Anakin.