In my opinion you were rude first, because the poster was talking about their (assuming beloved) pets. Posting about feeding rats to your snakes was rude. It's askin to someone responding to you posting about your (again assuming) beloved pet snake with a comment about how they killed a roaming python with a shovel.

Most people with a snake tend to think of rats as "feeders", not as pets. But to someone keeping rats as pets, they are as personable as a pet dog or cat. I think they had every right to be upset with you. I'm glad you apologized, but it doesn't take away the shock and horror of you posting about killing their chosen pet species so casually.

Just my $0.02