I think Aang taking away Ozai's bending made perfect sense. Ozai would have been absolutely revered in Fire Nation society, for the most part. He had completed the hundred-year quest to fulfil the national dream of conquering the Earth Kingdom, expanded the territories of the Fire Nation, and was an exceptionally talented bender. If the Avatar were to kill him... that's 'taking sides', so to speak. How do you repair the situation after that? You think the Fire Nation is just going to pack up their toys and go home? No - they have Ozai's brilliantly gifted daughter Azula, a clear successor; and what's more Ozai will now be a martyr. There can be no peace when the Avatar has the blood of the Fire Nation's greatest on his hands.
Equally, Ozai can't just be captured. As long as he's alive, there will be loyalists. Look at the real life example of the White Army after the October Revolution in Russia - monarchists continued to fight even after the Tsar and his family were taken into Bolshevik custody and the situation was de facto irreparable. The only way you can reach any resolution to the kind of war that the Four Kingdoms had been involved in is to 'defang' Ozai. The Fire Nation is intensely militaristic, and the concept of being lead by someone incapable of bending is anathema to them - one of the reasons Ozai was so disappointed by young Zuko. If you keep Ozai alive, he's not a martyr, but if you remove his bending, he can't lead. It's a destruction of the very psyche that powers the Fire Nation. When that is combined with Zuko's defeat of Azula, Zuko at last becomes a credible successor figure - a firebender capable of, as far as the Fire Nation knows, defeating the exceptionally gifted Azula, but with enough of the old lineage to be accepted particularly as Ozai is still alive and his father, giving Zuko an air of the prodigal son. None of this happens if Aang kills Ozai - the situation would probably have become much worse and taken much longer to resolve.
My only regret is a) how none of this was really shown to the viewer, and b) how light-hearted removing someone's bending is perceived.