At the start of a new campaign, I require all PCs to be Good. After that, their alignments can drift if it is appropriate to the character and to the campaign. If the PC becomes Evil, so be it.
During a campaign, a new PC might be added to the group. Such a character can be Evil if and only if every character in the group is at least Neutral and at least one character is Evil.
Additionally, I should note that there is a wide range of 'Evil'. Some Evils are entirely compatible with characters of other alignments, while others most definately are not. The former are allowed; the latter are not. The difference should be obvious.
One of the most satisfying PCs in a campaign I ran was a Dwarven Cleric of their god of justice. This Cleric gradually became more concerned with law and with results than with any notion of Goodness over the course of the campaign. He thought nothing of resorting to torture, or even killing, if he deemed it appropriate and necessary. As such, he was clearly Evil. And yet, the character was dedicated towards a Good end, and stood in direct opposition to my BBEG. As such, he was entirely compatible with the goals of the group, was friends with that group, and caused absolutely no problems for me, the player, or the rest of the group. I would unhesitatingly allow such a character again.