Panel 1: We see Liz carrying a load of groceries when she's approached by a silhouette that she identifes as Warren.
Panel 2: He offers to help her in with the groceries. Her "OK, but" means to me that he'd better get lost soon. To him, it's a fresh chance to start over and make her his.
Panel 3: He then apolgizes for his sudden arrival two months ago but he had to see her. (Followed by carnally knowing her.)
Panel 4: As he launches into a bit of self-flagellation which, knowing him, he hopes will get him laid, Liz tells him that she's engaged.
Panel 5: As he sees the ring, we see that "Dracula-meets-crucifix" reaction we were warned about. Obviously, he has no idea that Liz had someone in her life who wasn't him and the shock horrified him. I'd sympathize but he's just as big a douche as Nostache.
Hypothesis: I think this answers a question his last appearance raised. We all asked ourselves if Liz actually told him about Anthony and the life they'd built? The answer is "No". She punked out and stupidly, inadvertantly let him think he still had a chance. She pulls stuff like this all the time. Anthony knows about Warren but not Paul. Warren and Paul know about each other but not about Anthony. Her being coy makes her look like a mean-spirited little tease.