are a disease to society.

I'm not saying it's right for them to be jerks to everyone around them, but they're probably not prepared to die yet, and they're angry that they can't control their fate. It's kinda like the stages of grief except they're stuck on the anger part and haven't moved to acceptance yet. In addition to that, they probably don't want to show vulnerability and show the people around them that they're actually scared of dying since, like you said, everybody dies anyways, so they try to cover it up by pushing others away.

I'd blame the dad for spoiling him. But I guess it's only natural for a spoiled, rich kid born into privilege to act that way. Well, he'll eventually have his own character development in the end.

Keeping that up for another 7 years is somewhat admirably... or not. Ho well, let's see if he's been like that to Sora as well (the little snippet in the city traffic insinuates otherwise) or if he decided to be an utter despicable prick only towards everyone else.
(Careful, might get you stabbed without any involvement of an angry mountain god whatsoever)

Uh.. yeah, without giving you too much spoiler, he is like that to Sora too, but I have a feeling he is acting more like a tsundere when he is purposefully being mean to Sora (behind that facade he actually cares for him). But that side of him is benign in my opinion. What is frightening is the violent outbursts that he sometimes has due to his affliction. You will see how bad it can get in a few chapters. Stay tuned...

If that was what he's doing I'd disagree with you. He's just using it as an excuse to treat others like shit. After all, in the scenario you've given, no one around him is deliberately causing him pain but he is deliberately causing pain to those around him. But I don't think that is what is happening. He's being a shit to protect OTHERS in the only way he thinks he can.

It's not benign if you're causing anyone pain for no reason.