The cast of Lost actually said when someone dies on the show the actor doesn't leave because you never know who's going to come back, who's going to be in a flashback, who's going to show up and talk to Hurley, etc. The Charlie in Season 6 didn't ruin him for me because that's how he was before he came to the island. He had to get clean on the island because of Claire, with Locke's help, but in the flash sideways where the island never happened he had no reason to stop. But I think when they got to the church and passed on he couldn't have been shooting up anymore.