Paul Farquhar
Legend
Villains who the players never meet are boring to the level of pointlessness. The trick is to make sure the players don't know the character is a villain. or to make sure they can't kill the villain without also killing the good guys, and therefore losing anyway.Step one.... never ever ever ever let him meet the group in person until you are ready for the final fight. Sendings from a far, notes, the impact of his deeds noted through secondaries....all good.
But 5e is not forgiving to single monsters, you have to assume that should the villain and heroes ever be in the same room, the heroes will attack them, and kick the crap out of them. Let it be only one fight....the final fight.
It doesn't really matter if players do unexpectedly kill a villain. They were just working for someone else, or it was a clone, or somehow, they returned....