Seriously, why is he evil? I mean, sure, his throaty voice and his penchant for laughing maniacally as he disappears into thin air doesn't exactly give anyone good vibes, but the guy helped the heroes through and through and spoke nothing but the truth.
And his intentions are noble; sure, he wanted power, but he also wanted to elevate all of mankind into nephalem by sharing his power. Once all mankind have become nephalem, then humanity wouldn't have to worry about The Balance or the Burning Hells or the High Heavens or all that shit; they would have been powerful enough to present an equal if not greater threat to both the burning hells and the high heavens. Humanity would become a superpower capable of defending itself, instead of having to rely on a bunch of random heroes every few decades.
This is infinitely more noble than what the angels did to mankind, where they put a giant stone underneath the mountain that sucked all of mankinds powers and left them weak and vulnerable. Heck, they even almost decided to slaughter all of mankind if it weren't for Tyrael vetoing off the decision. Zoltun knew that the angels were supremacist assholes whose only intention was to keep humanity down, so he made the black soulstone so humanity could become powerful enough to stick it up to the angels. And how was he rewarded for this noble endeavour? He was hunted and killed by his own Horadrim bros, under orders of the angels.
During a quest, Zoltun Kulle even warned the heroes early on that their allies are gonna fuck them over, which really happened. In fact, the only reason he didn't give the heroes the black soulstone is because he knew something was up, and that giving the black soulstone to the heroes was going to make matters worse, which it did. If the dumbass heroes actually listened to Kulle, Leah would still be alive, Diablo wouldn't have been ressurected, and the invasion of the High Heavens wouldn't have happened. Zoltun Kulle knew this was gonna happen, but the heroes were too stupid and arrogant to listen to reason.
And even after all that, he was nice enough to give the heroes one final warning and leave the black soulstone alone. Heck, he could have just destroyed the heroes without any notice, but Zoltun Kulle was gracious enough to not only let the heroes leave unharmed, but also offered to share his power with the heroes, probably as a token of gratitude for reviving him. And what did the ingrate heroes do to him? They slaughtered the poor man and stole his blackstone Kulle sacrificed so much to create. These aren't heroes, these are thugs.
So Kulle really was a good guy after all. If you're looking for a real asshole, then look no further than Tyrael; the whole clusterfuck that happened in Act 1 was entirely because Tyrael was an idiot who couldn't descend from heaven without stirring up armies of undead throughout the countryside. Seriously, it was mentioned that the only reason all the demons and undead ravaged the land was because they sensed his power, so all the suffering everyone endured during Act 1, including the death of the blacksmith's wife, is all Tyrael's fault. As if that wasn't enough, Tyrael could have landed at a more convenient place like at the centre of town or in the middle of an open field nearby, but noooooooo he has to land deep within undead-infested dungeons. And even then once you reach him, does he tell you anything useful or important? No he didn't, the guy was too stoned and drunk to remember shit, and you had to jog cross country and kill more zombies and monsters and spiders and goat people just so we could get his sword so he could remember. Holy shit Tyrael was a huge fucking asshole.