Well, to be honest it was one of the only two other names I ever saw given to him apart from Bakura which I just can not get behind.
Kazuki has always made characters look spot on like another if he wanted them related by past lives or with just a very minor bit of changes to them so to me Thief King is not a true Bakura but rather some other unfortunate soul lured into working for Zorc. (This is cleared up more in the manga when the hold over him is severed and Thief King screams in panic having no idea where he is and what is even going on.)
That aside, the name was first used on a Japanese website by fans so when we finally got ahold of it that was what was used in fanfictions all over and how I got my hands on it.
I think it’s great, has a personality of it’s own, and overall is much easier then saying Thief King every single time when refuring to the bandit.
I also made some friends so time ago, one who favored Akefia and called him by that too, my other friend into Bakura, and me into Marik so we were the PIC for a while. I miss them but they broke off from the fandom to go better find themselves and I wish them nothing but the best of good luck and should they look back that they remember our time together fondly.