One can understand, as Ralph Fiennes tells the Evening Standard, how that could get to be a bit much if you were just going from one job to the next and playing somebody else evil day after day.
For a certain style of actor, to play a role well you need to understand the character.
Eventually, one might just want to take a role in a nice comedy for a change.
When the character in question is a homicidal nutjob or a World War II-era Nazi, you have to understand some fairly terrible people.