Ravenous Bugblatter Beast of Traal
Sure, I have no problem with a character who starts the game good but has made a pact with a fiendish power. The fiend is going to string them along and try to tempt them at every turn. But if they continue to resist the Fiend and use its power against itself willfully, eventually that Fiend is going to stop giving them more power.
I have no problems with a good character making a deal with an evil parton either out of dire expediency ("our village will be wiped out in the flash flood") or ignorance ("he promise me power to do what I thought was right, little did I know that he'd try to warp that sense of right over the years.") In general, I do see patrons have have immortal-level patience, continually tempting, treying for the small pebbles of easy that slowly turn into an avalanche - I'm not going to default to telling a player that by the time they are mid level that they will be stripped of their powers. Rather work out with the player to tell the best story we all will enjoy - which might be the stripped of the powers, or something else entirely up to the point that the patron keeps granting for the length of the campaign, always looking for that 'first slip'.