What bad decision did they make?
Bioware doesn't bother explaining what lead the mages to pledging themselves into servitude, probably because they couldn't think of any real reason that wasn't utterly ridiculous. I think that was a really huge plot hole. I mean if we assume that the mages were reasonably intelligent, then it should have been obvious that the Tevinter mages were scumbags who couldn't be trusted. That guy was the most obvious villain since Seymour summoned Dagon from the Underworld in Final Fantasy 10. And there is no real doubt that the Tevinter empire is completely evil and corrupt.
And what were they even gaining? Maybe the help of 2 extra mages? 1 of whom was suffering from cancer or corruption and so wouldn't have been of much help anyways...
And once he had the mages, why did he continue to stick around in Redcliffe for no reason at all? Why didn't they all high tail it back to Tevinter then and there? Instead they hang around Redcliffe for weeks on end with no real purpose while their enemies gather together to overthrow them.
If we want to dig even further, why are the only people who are even aware of the altered timeline those in Redcliffe? Shouldn't everyone's memory of events have been changed the moment that the Tevinter mages go back in time?
Also we need to consider how the mages were all but coerced to pledge themselves to Tevinter, who procured an agreement through a mixture of fraud and force.
We also don't know how the mages came to occupy Redcliffe or if the mages had been welcomed into Redcliffe where they remained peacefully until the timeline was altered at which point the narrative got lost anyways.
Ultimately the mages seem to be innocent of any actual crime, the only crime that was committed in Redcliffe was poor storytelling.