Bret screwed Bret.
I believe the terms of the contract were "reasonable creative control" during his exit period ... refusing to drop the strap to whoever Vince wanted to put it on is not reasonable. To say, "I don't want to be squashed, I want it to be competitive or even, since he's a heel how about if he wins it with outside help or some sort of cheating" is reasonable -- to say "I'm not going to make him champ" is not.
Bret chose to take the WCW contract. Yes, Vince walked him to the door and opened it for him, saying he couldn't live up to the terms of Bret's pie-in-the-sky contract with WWF, but Bret chose to leave. He knew Shawn was the guy who was going to be the next champ and he had personal heat with Shawn ... but it seemed as if Bret wanted to leave in such a way as to go to WCW and be able to say (or have people believe without him saying), "I was the champ there and I beat and am better than the champ after me" -- something VKM couldn't allow.
Remember that the trust issues were more with WCW and Bischoff than with Bret himself -- they had previously signed a Diva champ and had her throw the WWF belt in the garbage on TV, right? Vince had every reason to believe they might have Bret do the same and bury his company's biggest championship. No way was that going to be allowed to happen.
The real thing to me is that Bret seemed to be such a total mark for himself that it completely clouded his judgment. His gimmick is that he's "the best there is, the best there was and the best there will ever be" ... and he completely believed that was true. He believed he was a national hero in Canada (he was a popular entertainer, which is different than being a national hero) and therefore thought that meant he couldn't lose in his own country. That is utter rubbish -- imagine Ric Flair refusing to the job in North Carolina or someone else taking a similar stance.
I think it would have been reasonable for Bret to insist on not losing his belt in a way that would cheapen and weaken his character -- saying no to being squashed or quitting like a coward or wearing a chicken suit into the ring or cutting a promo saying Canada is a shit country, etc. -- but not where or to whom he was going to lose.
Bottom line, Bret was being unprofessional and thus screwed himself.