Let's count the ways...
In the culling of stratholme he disobey his prince, so on top of insubordination - he is cruel, condemning all these people to eternity of undeath instead of the quick release to the afterlife.
He is also Pridfull, not acknowledging the truth in Arthas actions (to that point) mainly because he is his student and doesn't see him as superior to him as a paladin.
He his furious and filled with vengeance for the murder of king teranas, and his own death by the hands of Arthas.
He can not ascend because he's so childly stuck on what Arthas did to him. He then cheats his way to asscention - and betray the kyrian covanent who gave him a chance in the afterlife.
Lastly, he lacks compation. He refuse to acknowledge that Arthas was first of all - his responsibility as his mentor. His failure. He ignores the jailer in the helm guiding Arthas deeds. He tosses a helpless Soul to the maw, denying it its right. To be judged - and calls it 'justice', not vengeance as it is whats it is. That also makes him a hypocrite.
Gotta say, I never cared much for Uther, but SL made him even worse on my list. Arthas is a villain, mostly influenced and manipulated by powers beyond his understanding. Uther is a Villain on his own with no one to blame. Same with Xera and Yrel, you can still yield the light and be a horrible person.