An awful player doesn't score 12 goals. I agree tha the wasn't up to his normal standards, but let's be honest, if Willian is your best player during a season and by quite a distance something is wrong with the team. Costa is a very good player, despite being a despicable human being.
Costa is one of the most ruthless finishers I've seen in a long time in the PL. That Liverpool finish (i think the first season) where he hesitated just a few extra seconds to smack it in was a beauty for anyone who admires the craft of a great finisher.