The way Ardyn behaves in the story is almost like he has not done anything wrong, even if he is the villain.
Most people say the Gods of Eos are the story's true villains. I may have to agree. They are right. Noctis and Ardyn are brethren. They are from the same line.
When it comes to Noctis's sacrifice at the end, and why he did it... There is nothing that we have to make us hate Ardyn. The Gods of Eos are questionable at best.
Ardyn is not that dishonorable because He had some good in him. Do not blame Ardyn for his actions because he is a victim. Blame those meddling Gods.