I love this exchange. It's always been the Howard way to go on the attack when he feels guilty about something. No, Howard's not responsible for Artie's bullshit drug addict behaviour, but his hands are not clean in any way.
One of my favourites is the endless harping on about Stuttering John after he left the show, while at the same time claiming "all he did was stutter." To paraphrase the joke man: "Either I'm worth the money and you pay me, or you say no and move on." You can't have it both ways.