Hey, the bag sizers are what they are. And UA's regulations as to carry-ons are what they are. The OP had a bag that was oversized, and s/he was delaying boarding by not once, but twice, trying to go through the line only to be refused and then stopping to repack the bag. If you were standing behind this guy in the boarding line, what would have been your reaction?

(a) That's OK, just try again a third time to repack your bag to make it legal? Or
(b) You go agent, make him gate check the bag and stop holding things up?

Maybe the agent was snippy, but the OP's behavior seems to be a bit over the top. If s/he hadn't overpacked his bag, and then tried multiple unsuccessful times to make it legal, and then decided to try to ridicule the GA for enforcing UA's rules, this wouldn't have happened.