I'm wondering why Imperius really hates the Nephalem in the Act 4 of Diablo 3... He even goes up to wanting to kill you right before the final boss.
I don't see why. I come to save the whole of creation, him along with it, and ending a war that has started at the beginning of times... That would make my day, any day, for me.
So why does he want to kill the savior of everything?
I presumed it was simply because he voted on the "destroy humanity" side of the original debate. I'd also guess that this was at least partly because the nephalem have the potential to be "greater than both" the angels and demons, threatening his power.
I don't have anything to back this up, though - just my own take on the situation.
As Raven Dreamer points out, there's also the immediate fact that he blames you for Diablo's resurrecterific rampage through the High Heavens, which is probably plenty to annoy him on its own.
Another factor of Imperius's hate is that he believes in the order of things. There has not been a change for a long time, so he is very indifferent towards everything unusual. As Melini said, he is disgusted by Inarius and Lilith's love and is restless because Malthael, in his advisory role, is unable to predict the future of the Nephalem.
Imperius believes that the Nephalem are a sacrilege in themselves which is why he is not so happy about us, the player, walking into the gates of heaven, right after his failure in the final battle against Diablo.
If anything, we can be sure he will be hating us more in the possible future expansions.
If you are intereseted in a more detailed explanation I'd suggest you read the sin war novels. lilserf is right and I can only back him up. Imperius is battling demons since the beginning and hates them to the core. He fears the demonside within "us"/humans/nephalem and would rather fight demons than hybrids since he knows his craft rather well(beeing angel of valor for a reason).
Also, he is simply disgusted by the idea of the angels mixing with demons and disgusted by the treachery of Sancturios birth so to speak: "HOW WILL YOU CHOOSE, YOU WHO HAVE SEEN THEM AT THEIR MOST FOUL? WILL YOU VOTE WITH ME AND PUT AN END TO THIS MOST HEINOUS OF SPAWNING GROUNDS?" (an exerpt of Imperius talking to Tyrael and the council during the vote for Sancturios destruction)
