thing is that his moment towards tabia was more or less the only interesting part of him,where he really questioned if he should stop with what he had done.
The new lore keeps that aspect with Azir,where Xerath realises that he had done a mistake,he'd like to go back,but the preparations he did where already too far and couldn't be stopped anymore.
What I think could be possible that xerath originally wanted to become free,but he needed power,as in magical power(because as a slave it is impossible for him to obtain political one),thus leading to his obsession with pure magical power,his desire to get more and more of it so that no one can ever imprison him ever again.
Another possibility is that his obsession with power came after his imprisonment,I mean he was imprisoned for millenia(?) with renekton,he drove renekton mad,but I doubt that Xerath himself didn't change in all that time.
Also Xerath did manipulate the ascension ritual,i mean he not only stole the ascension from azir,he improved it,he holds so much power,that 
couldn't kill him despite the fact that he had millenia to do this.The sun disc was a tool and Xerath learned how to bend it to his will.
Well, Xerath, as all champions, had only his lore and judgment as pieces of story in which he was portrayed. So to say that the moment with Tabia was the only interesting thing is the same as saying that the only thing they wrote about him and told about him as a character was good =P
The point is that there is no real moral tension in the new Xerath. He doesn't really doubt himself. He's more like an angsty teen seeing the whole world warped through the lenses of his supposed misery. Because he wasn't mistreated, he didn't lead a wretched life.
And when the moment comes in which he should find himself wrong he just goes "too late, bro".
I don't think the new Xerath "lives and breathes" as the lore team would like to say. He's a slave, wants freedom, sees the Shuriman society as warped, but he doesn't hesitate to plan a mass murder rather than maybe exercising his influence to actually change Shurima. He is blind and one-dimensional. He doesn't care about other slaves, he's selfish and just angsty. He is more like a child denied a toy long-sought. He doesn't even really need freedom, he is more powerful and influential than most. And he could just have spoken with Azir and found common ground.
Come on, years plotting, bribing people and tricking Azir into Ascension and he never ever got any glimpse of what Azir thought of him and all? This would mean he's not as clever as the whole scheming part would suggest. He actually turns out being pretty dumb and monochromatically evil.
"I was a slave, so I opted for genocide" is a pretty lame reasoning and if he actually regretted planning it all and realizing too late Azir's true intentions then why is Xerath theme all about power? Why all the in-game rumbling about power, true magic, the will of man unbound by flesh?
He wanted freedom from his society, there was no prior desire towards freedom from a mortal shell and since Shurima is no more why should he still be so hell-bent on power? He should be a character about regret of the evil he has done and the doom he brought at the very moment he finally received what he sought and thought fair.
Simply put, they changed the character and now we have a backstory which is not befitting of the magus archetype he should represent in the game.
