I think Qui Gon felt Obi Wan had come to terms with Bruck's death and misjudged his ability to deal with it. Truth be told I suspect Qui Gon felt he had to meditate a great deal on the death of Xanatos. Xanatos after all was his foe and best friend for equal amounts of decades and the suicide of a man on your account can seriously cloud thinking. As for Obi-Wan I think Qui Gon was giving into Fear. Fear leads to Anger (Revenge) Anger leads to Hate (Balog will taste that no doubt) and hate leads to suffering (Everyones) Qui Gon by this point I'm sure had let his emotions be out fo control and he was so worried about Tahl he had subconciously given up for a loss.