Bran could have told Theon...

Maybe Bran is evil.

Hmm.
Maybe his precognition doesn’t work that way. There have been other examples in fiction where the precog could SEE what happens but not CHANGE the course of events. So all he could have told him would’ve been: "Oh, by the way, Theon, thanks for the effort, but you’re going to die in a few minutes."

if Bran told Theon he didn't have to die, that would change the course of history and cause all mankind to die off.