I think you're wrong because Merlin didn't try to kill Mordred, he merely tried to allow the witches to succeed. His error was in misinterpreting what they were doing. When Gaius said to him he was the only one who could save him it was clearly wrong since Arthur already knew there was another way and there was no guarantee Merlin could have countermanded the spell. It's not like he's all powerful or perfect. Nor is Arthur. But between them they could make something mighty.
They were never supposed to be perfect. Or wise. They've blundered around for 4 seasons but now, at last they're becoming wise. It's probably too late but that doesn't make it any less gripping.