"Justify your backstory!"

"A gnomish paladin tinker made a Helmet Of Reverse Alignment and then started summoning devils, and using his one order, made them put it on their heads."

"...fine."

And then you do the whole campaign having the devil remember being evil, remember enjoying it, but not enjoying it anymore, and the crisis of faith that brings on. Every so often he will torture to death someone, just to see if he can recapture the feeling of evil being good, but it only leaves him hollow and filled with regret.

It's not about doing things they want to do, it's about making sure their enemies can't do anything at all.

Then it would not be a devil, trying to make this work in to campaign is either pretend it is nothing despite making no sense or have it be a major part of the campaign, an event that changes the very laws of the universe is not something that is unnoticed by gods, demons and devils. A player wanting to do that means to me that he is an incredible narcissist that I would not want to play with and I would not allow it as a GM.

Now as an NPC it could work, is actually something that gives me a few ideas to center a campaign around. Granted it would be something that the players would have to realize later in to the campaign but it could work as a higher level campaign. Depends a lot if the players want a game with high stakes or not. But again, it would be this world changing event the very existence of this non-evil devil.

As a parallel, Drizzt was a drow that was not evil, while I do know they later made drow not inheritably evil, in the books the reason Drizzt was not evil was do to genetics so the drow are evil do to the corruption of Loth but it had this rare mutation that was unheard until Drizzt, technically his father. Drizzt is this amazing character do to him being unique and rare. A player character that is drow, while not that uncommon now, it would need to play an important part in the campaign, that is why I'm not a fan of drow player characters.