I get that, it still doesn't make sense to me. There isn't a single deed or action he's done that makes him worthy of becoming a disciple. Rhulk razed his planet and was the extinction event of his species. I can't really speak on Nezarec as I fucked off from Destiny during Plunder and never went back to finish the story when I returned for Seraph. Calus? He's a glutton who wants to indulge in opulence and be the witness of the end. Even leading up to Lightfall his two major plot beats are presage where he tries and succeeds in communing with the Witness and Haunted which is him trying to break Zavala, Caitl and Crow by using their past failures to torment them through the power of the Lunar Pyramid which is explicitly Nezarec's. Maybe he could've been an abomination in the form of a Subjugator mixed with a Tormentor with the Witness telling him that he will grant him his original form but as a disciple making him all the more desperate to succeed and crush us. Then at the end he is transformed into his proper disciple form so us crushing him is all the more humiliating with the witness laughing as he falls.
I think that's the point of it, honestly, that Calus
wasn't worthy. Calus led a life of hedonistic pleasure, only to be deposed by his more militaristic advisors and exiled from his empire. He drifted aimlessly across the stars until he somehow brushed against the power of the Witness which gave him a purpose again, although whether that was actually the Witness communicating with him or his own delusions is up for debate. Even then, his decision to become the Witness's herald was self-serving, as it was his ultimate goal to be the last one alive at the end so all would know his glory.
Out of any of the Disciples, Calus had the shortest tenure by far, and that was likely by design of the writers, showing that it was his continued vanity and greed that proved his undoing. He kept begging and pleading to be noticed, and it was only after he communed with the Lunar Pyramid that he finally got what he wanted, unlike other Disciples that the Witness approached first. It's likely that the Witness could see that he wasn't actually a true believer in its cause, or at least not in the same way, but after the events around the Moon, it saw that he could still be a useful tool. To that end, though, the Witness really didn't seem to care about Calus succeeding or failing because it had contingencies in place either way; all of the Disciples are expendable, of course, but Calus especially so. He even flat out yells at Calus halfway through the campaign because he clearly doesn't understand his place and continues to believe himself more than a speck in the face of a god. I think it's more poignant that the Witness says absolutely nothing about his death, showing just how insignificant he truly was, adding to the overall tragedy of what he had pursued for so long.
There are a lot of ways it could have been handled differently, sure, but I don't really have an issue with how it all played out. Except for, as mentioned, the end.
