Hades coming back from the Isle wound up being frightening for all of his siblings, even Hestia, because they have never seen him that angry and upset before.
Sure, Hades has his moments of temper, sometimes quite legendary ones, but this was so much worse than anything they'd seen from him before. Hades doesn't have freakouts as bad as this- this is like, a Hera level freakout, maybe even a Zeus level freakout, and seeing Hades, who's usually got the most even temper of all of them except Hestia, angry at them to that level was unprecedented and terrifying. They were genuinely afraid this was his villain origin story. That he'd been pushed too far and now he was finally turning evil and was actually going to start trying to destroy Olympus. It was that bad. So bad that they thought Hades, who's never expressed anything other than disinterest and vague disgust at the idea of being on the throne of Olympus, was going to try and overthrow Olympus. And not even out of an actual desire to rule. Just out of spite.
They'd known Hades would be hurt. They expected he'd be emotional. They'd assumed he would be some level of angry. But that? Only Destiny herself could have seen that coming.
And somehow, that was the easiest part of his return. It only got worse once he calmed down.