<p>I'm surprised Korra didn't try purifying the whole mecha suit once she discovered it was powered by spirit vines. When she tried it on the Republic City vines in "Change", they reacted pretty violently. It could have saved Mako from needing to shoot the vines full of lightning and probably would have meant Hiroshi lived. </p><p>But having Hiroshi die was a good way of getting Korra and Asami to bond a little more, so there's that :P </p>

<p>It was a humanizing gesture by the writers. Hiroshi Sato, for all his villainy, is at least relatable. He was once an inherently good man who suffered a horrible tragedy when his wife was murdered. His anger now directed at benders in general, he committed some terrible crimes, not the least of which was nearly killing his own daughter. Prison can either harden or humble a person, and in his case, the writers chose the latter. Turning him into an unrepentent bastard would just rehash what had already be done with Zuko and Ozai, so we are given an admittedly forced scenario in which Hiroshi asks for forgiveness and Asami is reluctant to give it, and understandably so. His final appearance is one where he gives his life to save the city he once tried to overthrow, and saving the daughter he once tried to kill. Does this justify his previous actions? No. Does he redeem himself? That's up to you to decide. </p>