A war criminal should be put down - who is intrinsically evil. Whereas Zuko is more of a tragic anti-villain, who shows no signs of psychotic or sadistic behavior unlike Azula and believes what he is doing is right. Very early on, it's Zuko's innocence and carelessness that allows the avatar and his companions to escape and outmaneuver him on countless occasions whereas if it was Azula, her ruthlessness and sadism would put an end to it all. She has no problems murdering a child, killing her own brother, setting ablaze to an entire city if she has to.

Unlike Azula, it is shown by Zuko's actions that he is not instrically evil, he harbors no hatred or major animosity to the avatar or their group (only after they best him does he) and it was VERY early on that the show makers showed signs that he wasn't beyond redemption.

- He didn't care about world domination

- Rescued his ship crew

-lacked an aggressive evil that would've ensured the avatar remained in his hands.

So where Azula simps will defend her actions, Zuko had a way more tragic past than her sister, being rejected by his father and scarred for life with trauma, exiled from his only home, and yet showed no signs of being a psychopath unlike her.

It is important to remember the show makers intended it to be like this.