Sorry to hear this. I thought Karlen was great.
In fairness, Willie had a lot to cringe about. When he wasn't getting his blood sucked Barnabas was smacking him about with his cane. Returning good for evil, Willie was forever trying to talk Barnabas out of some nasty and reckless course of action, since Barnabas' first impulse when dealing with a troublesome individual was to kill him.
He certainly looked younger than his mid-30s on the show.
Obits:
Hollywood Reporter
Variety