While yes, It would be possible to construct an "evil" guild, Morality in the real world doesn't work like it does in writing or tabletop RPG's, and working under someone, no matter their ranking power they have isn't an inherently evil act. It's important to realize that what we consider good/evil is based on our own perception and understanding.
for example, a race of undead beings who worship death wouldn't think of their gods as being evil, But those who worship life would.
Or, More realistically, Robin hood wasn't evil to the poor, but the rich would have seen him as such
Throughout history many kings and rulers have had what you've described, A group or person to deal with things less savory. This doesn't make their actions inherently evil.
If you want to do the occasionally morally bankrupt thing, robbing, stealing, murdering, etc. this still wouldnt mean you were "evil" persay. Its not like fipping a coin.
If you want to have an "evil" guild, you should focus on finding a king/ruler with "evil" goals; doing the occasional odd job for a king who is a paragon of virtue, who just needs the occasional bandit discreetly removed would still make you a good guy.