He was what the gods' machinations had made him. For all their idiotic yammering about fate and predestination, every single thing that happened of note in the main story happened because of either their direct intervention or their machinations. If anything, the real villains of FFXV are the gods. Ardyn was evil because they manipulated him into eating a tonne of daemons, gave a vision of him ascending the throne to Aera, likely knowing she'd tell Somnus because she's an idiot (I'm sorry, Aera, you're a nice person, but what person with a brain in their head trusts a man who literally BURNS PEOPLE ALIVE?), then knowing that Somnus would be exactly the kind of brazen sociopath that would try to kill his brother and anyone that got in his way, let shit just kinda play out like a particularly psychotic soap opera.
Somnus' whole "oh, I got better and leanred the error of my ways" spiel might've rung just a little less hollow if he hadn't been, ya know, burning people alive and hadn't cut down (showing no remorse after the fact) Ardyn's fiancee right in front of him. "I became a better person" is what you say after you steal $20 from someone. That doesn't cut it when you were a fucking serial killer.
Thing is, I'm certain Ardyn would've gladly sacrificed himself after becoming the focus for the Starscourge had the gods not arbitrarily decided to wait over 2000 years doing fuck all. Yeah, he'd have been upset and rightfully so, but back then, he was still a fundamentally good person and would've given his life to save everyone from fucking daemon malaria.
Every god on Eos deserves to be rounded up and summarily executed for their actions. Not just this, but that little internal tantrum they threw when the Solheim civilization stopped sucking their dicks with enough gusto and they fought a war that ended in complete (and very convenient) genocide. Is it any wonder they fucking built Omega?