As others have said, it'd depend on the boldness of the evil.
One character I played was an absolutely remorseless killer and assassin/bounty hunter. He would not kill for no reason, but he would coldly kill anyone who was less trouble dead than alive. This could include random townsfolk or even PCs.
However, he included the hassle of dealing with pissed off party members and armed city guards, as well as the potential posses, in his estimates of "less trouble dead". He also understood that, as a hired killer, he was marked as one of dubious trustworthiness. The only way to establish his credentials was to always deal squarely with people and never turncoat for more money.
Finally, he was smart enough to realize that a lot of evil extraplanar entities would not make good additions/rulers of the Prime Material (sorta "That's where I keep all my stuff!"). While he would accept various small jobs from them, he was very willing to oppose their grand machinations pro-bono (although he never turned down profit).
The end result was a character that actually worked quite well with good PCs. The character would rarely risk destabilizing a group that was helping him meet his goals (good living, basically). I considered him evil, rather than neutral, because he really had to moral objection to murder. Pure pragmatism is what kept him in check.