No, they're not too dumb. Rats are actually very intelligent animals. You just got lucky that it didn't try to bite your snake, but you did the right thing by being prepared to assist in the event of a bite. I don't think there's any reason to prekill your rats if you're supervising feedings like you are. Just be prepared to push the rats mouth away from the snake if he gets a bad strike. (I use feeding tongs or a pen or whatever I have handy in that situation)...