Mm, you're paintin' a picture. And I like it.
Magni Bronzebeard is the brother of Muradin and Brann, and was the King of Ironforge 'til around the Cataclysm hit. When all the elemental unrest and earthquakes and volcanoes started going off as a result, Magni turned to an ancient spell that was supposed to make the caster 'one with the earth', hoping this would allow him to settle the elemental unrest and preserve Ironforge. The spell succeeded, but as a result Magni was turned to diamond and became a frozen gemstone statue, unable to move or speak. It isn't until Legion that he regains the ability to move, having apparently been communing with Azeroth's worldsoul in the meantime and had his mobility restored so Azeroth could communicate her wishes to mortals.
There was a scenario pre-Legion that covered some of this, and another scenario unlocked in 7.2 that touched more on it.
First time I had my doubts about him was when Azeroth sends him and us to Argus to be rid of Sargeras, probably the only being currently known that is a direct threat and opposition to the Void.
I know she's supposed to be the most powerful Unborn Titan, but the fact that she knew how things would transpire, something that derives in occasion from Old Gods it's suspicious. That whole scenario is sketchy.
I wouldn't be surprised if the "Heart of Azeroth" turns out to be a device that funnels the Azerite's energy into N'zoth. It would be a very "Nzoth-thing" to do.
This is WoW, anyone and anything could be corrupted by any number of things at any time. Almost every villain in the game was someone nice until being re-written as someone completely different because they got corrupted by fel/shadow/aliens/voodoo/fire etc. Why bother writing good motivations? Nah, this dragon was nice, but then someone set fire to her and now she wants to murder everyone because... evil.
I don't think he is corrupted. But if the story goes that way, i think he may be manipulated. It may be plausible that the old gods had found a way to emulate Azeroth's messages and trick Magni that way?
He currently is Azeroth's protector. If he sees us as a threat during the course of BFA, to what extent would he be prepared to go to stop us with him being under such manipulation?
"Not everything that can be counted counts, and not everything that counts can be counted" ~Einstein
Wish more people would take that to heart.
corrupted, greatly unlikely.
manipulated? almost a guarantee, but odds are everything in the known universe from Magni to Illidan to Varian to Sargeras and the Naaru have all been manipulated into working for the Void Lords. even those that fight vehemently against them are PROBABLY working for them and just don't know it.
In WoW, every major Lore character will be corrupted at some point it seems.
Its a recurring theme.