There are a lot of theories, but there isn’t much concrete evidence to say for certain, imo.
It seems there are multiple questions that people want answered, to know for sure :
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When exactly did the Arbiter break down?
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Who personally did it/over saw it?
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What method did they use = what was the red bolt?
For question 1, it seems to be after the Emerald Nightmare Raid, and before Sylvanas breaks the helm at Icecrown. Maybe someone can narrow it down further.
For question 2, most people seem to think it was Denathrius himself. Others think it was the death of Argus.
For question 3, it gets more varied.
Some people suggest Denathrius just lobbed red anima as a weapon. The Warlock Venthyr ability is basically lobbing a red projectile. I see why folks suggest that.
Others think Denathrius unleashed Remornia. They point to the battle pet and one of its abilities as a clue. I suppose that is possible. But wouldn’t the Arbiter have obvious wounds or damage? Wouldn’t someone see the wounds and say : “it was attacked with a powerful blade.” It seems like the Arbiter was in good shape, but just offline. Like either a short circuit or overloaded.
And others believe it was the soul of Argus. Part of a plot, or just a fail safe. Perhaps Denathrius had planned this eons ago, when his minions in the Legion first made him aware that Sargeras controlled Argus. Maybe he planned to sabotage the Legion, and his plan was to lure the champions of Azeroth to slay Argus, capture his soul before it gets to the Realm of Order it may have gone to, and send it to the Arbiter. Which would either overload the Arbiter from the torture Argus went through, or because it was a powerful Titan soul she was never designed to process. Argus does drop a Red Scythe on Mythic.
Personally, I think it is still a bit up in the air exactly who did it, and how. Others are more certain.