Whenever I start a new playthrough of New Vegas, it always starts with me listening to what Vulpas has to say, then adding what's left of his carcass to the pile on top of his goons'. Nevada needs fixing and cleaning, yeah, but we're starting with you, you guys are not the answer.
For an actual answer, I'm taking the XIV slot (again) but with spoiler territory. >!Emet-Selch. After so much near cartoon villainy from the Ascians, Emet realizes that it might actually be a good idea to work with you and eventually reveals the reasoning behind the Ascians wanting to unbalance all worlds and resurrect Zodiark; there was only world and one people. When a calamity came, they prayed for someone to save them, which is when Zodiark came into the picture. Half and then another half sacrificed themselves to him and he saved their world, but then came a fracture as there were those that thought it not right to sacrifice so much to him. This is where Hydaelyn came into the picture. There was a battle and ended with Hydaelyn defeating Zodiark. She then broke his power in half. Then half again, and half again until it was broken into 14 pieces, along with the planet and people. Otherwise known as the Source and the 13 shards, at least until one of them super fucked the 13th.
Which leads into their goal; Emet-Slech and the rest wish to see their world and people returned. No matter the cost. Even if it meant millions upon millions would die. Emet himself just being broken after waiting so long and knowing he's the last of two of their kind, the third having died some time ago (because he was an idiot) as the rest are just shards like everyone else. Not only that, but every time he makes a push, someone (us) stops him. Which lead to him working with us and testing us. We do fail, but we pick ourselves up and prove the people he saw 'things' have a right to live and exist, leaving him to just ask us to remember him and his people for what they were.!<
Is the cause just and sympathetic? Honestly? Yeah. Was he sympathetic? Yeah. Were his actions just? God no. But people think he was right for some reason.