I also didn't remember getting the impression that Unukalhai had deliberately killed Regula - but my alt happens to be up to post-Heavensward so it seems to be a good time to go back and replay for a second take on it!
And in conclusion.... I would dare to say that Moose's quotes, while correct, are misleading in conjunction with claiming that Elidibus "specifically tasked Unukalhai with getting Regula killed one patch before he showed up in 4.0".
Unukalhai was attempting to kill Regula at one point, but I don't think it was ever stated that it was a direct order from Elidibus - he may have been acting on his own. He admits that he acts more emotionally when the issue of primals is involved, and it's possible he's going after Regula as part of his private crusade rather than Elidibus's orders.
More importantly, there's a gap between those quotes and the circumstances that actually led to Regula's death, which don't seem to be Unukalhai's doing or intention.
Our interactions with Regula, in context:
Patch 3.2: When the Bough Wakes (Sephirot)
After beating Sephirot, the WoL, Krile and Unukalhai are leaving the facility. Unukalhai (at the back of the group) hesitates and turns back as the others walk off, and Regula appears. He has evidently been tracking us while we went after Sephirot. Unukalhai gives his "you Garleans never learn" speech. Regula isn't interested in fighting him, and walks off.
The others rejoin him, and he deliberately doesn't tell them that Regula was there.
Patch 3.3: The Fate of Stars (solo mission)
The Garleans have broken through a ward placed over the Aetherochemical Facility, and we need to stop them before they get their hands on critical Allagan technology.
This is where Unukalhai explains about the Genesis Expression, and proposes that we have the opportunity to kill Regula.
During the confrontation, this is first where [one of the Scions] mentions to Regula that the eikon-trapping technology may be dangerous, and not the solution he should be seeking.
Later, Urianger confronts Unukalhai about the day's events - the ward he had placed on the facility was too complex for anyone, let alone the Garleans, to have broken so swiftly without a power akin to the Echo - and thus concludes that Unukalhai had both broken the ward and fed Regula information about what lay within. This is where he accuses Unukalhai of setting a trap for Regula... and then says he should have explained his plan to the other Scions so they could successfully close the trap together, and with less risk!
For now, Regula is still seen merely as an enemy.
Patch 3.4: Balance Unto All (Sophia)
The Garleans are not directly involved in this chapter, but we learn from Y'shtola that Regula was promoted on merit, and that enemy factions within Garlemald are eager to see him fail.
Patch 3.5: The Last Pillar to Fall (Zurvan)
We return to Azys Lla to learn that Zurvan's thralls have broken free and are running amok throughout the facility. Regula appears, declaring the Allan technology flawed and worthless to him, and offers us a truce - hoping that the intervention of those with the Echo will be able to save the lives of his men. (A dialogue option allows you to hesitate or refuse his offer, in which case Unukalhai and Krile volunteer first.)
Four generators feeding power to Zurvan need to be destroyed - the three Echo-blessed are necessary choices, and Regula volunteers as the fourth. Unukalhai seems fearful that he is not strong enough, and Regula tries to encourage him.
And so the critical moment: Unukalhai hesitates in battle, and Regula is dealt a mortal wound in coming to his defence.
Afterwards, he rushes to Regula's side.
"...You saved me. Why?" he asks.
And once we withdraw and regroup away from the primal's influence, he seems distressed. "He's dead. If only I had been stronger..."
It doesn't seem at all like he secretly intended for it to happen.
It's possible, of course, but I don't think it's true. He seems genuine.
Additionally one of his other driving forces is to atone for his failure to save his own world by saving this one - I think he wants to stop Zurvan far more than he wants to gamble his life on the chance that Regula will rush to his defence in time, and receive a mortal blow out of it, and that Unukalhai won't also die as a result.
So this perhaps is where people come away with different impressions of what happened. Yes, Unukalhai is accused of, and openly admits to, having plotted Regula's death - at one point. But for the immediate events within patch 3.5, his (and our) impression of Regula had changed, and I don't think he had any intention of it.
Also... "you're not alone any more - welcome to the Scions of the Seventh Dawn" says Krile at the end. Yes, the wonderful fellowship where we shut you in a side room and don't think to ask for your help while all your new friends fall into magically-induced comas.... Seriously, when are we letting the poor kid out? ;_;
And another thing....
When I typed out all of Ramuh's script a while back, I noticed that he seems to vary between upper-case "Light and Darkness" and lower-case "light and darkness". This quote uses lower-case, and he seems to be talking about man's actions rather than cosmic Light and Dark.
Assuming it was deliberate, I'm not sure that line can be used for theorising about Hydaelyn and Zodiark.