I see where those that are saying it isn't are coming from, however, considering that even thier responses are heavily "but"-laden, I would put forward that it is indeed poor GM-ing.
The only time a PC should really "become the villain" is when that player is finished playing the character, and the character becomes an NPC under the GM's control, making it actually a very intriguing and excellent idea. Otherwise, if a PC is the bad guy, What exactly becomes of the GM's job? Providing the bad guys and story is arguably the most important part of the GM's job (even if it always doesn't take the most amount of time in preparation) and this is disregarding the fact that, unless it's the "final confrontation" with the rogue party member (not necessarily the party's rogue the game becomes a cumbersome and incredibly boring thing for that villain PC.
...it would also be a good tool to use if you're switching DM's, and the villain "PC"'s player takes over. That would also be cool. anyway...
Disputes are one thing, but even in a party with a mix of evil PC's (some of them stupidly chaotic), there is always a method of making them work together. For example, my group of evil and unaligned players works together fantastically thanks to the fact that a god told them to face-to-face, with a very clear definition of what would become of them otherwise. Somehow, they're the "chosen ones" saving the world. It's quite silly.
The point being that as a rule, disharmony in the group is a good indicator of either bad GMing or someone needing to be booted (be it character or player). Though there are certainly exceptions (some of them awesome) to the rule itself.
I suppose the summation of my argument would be "It's poor GMing if you didn't plan it carefully, and/or if the end result ruins the enjoyment of the game for any player (9/10, this is the case). Otherwise, no, it's fine."
"Let's see... I think I'll toss a gold coin into that pile of filth those sewer-dwellers are digging through... "
"Okay... they begin fighting furiously, killing each other for the gold."
"But you're a priest!!!"