He neither had a rough childhood nor has he a broken heart. He has no excuse for doing what he does. He does not excuse what he does. He is proud of it. He delights in tormenting others, having no regard for anyone or anything. He is pure evil.
He actually does have a backstory: he is a dragon elder who moved on from hoarding trinkets and is now hoarding memories of the ones he terrorizes instead. This allows him to travel freely and grow even bigger than the other dragons. He spent centuries training himself to be in complete control of his greed, so that it wouldn't cloud his thoughts and 'ruin his fun'. He commits atrocities, then intentionally disappears for years until he becomes a legend. All he has to do next is rear his head and everyone who has heard the stories about him will be immediately frightened, increasing his size and strength without him having to lift a claw, since what he desires is to see the horrified expressions on other creatures' faces and not treasure.
Not any time soon. It's hard to tell when and what I'll draw, since I just draw whatever comes to mind at the time. I'm on a vacation right now and don't have my equipment, so it'll be a while before I post anything.
In the real world, maybe, but this is FiM. I doubt they do warfare like that It is also unknown if these things would even affect a dragon. Discord is one of the best weapons Equestria has against him thusfar, although he is a little bit of resistance to magic due to experience.
Cimmerian isn't perfect, but doesn't have a major weakness like, say, Archilles. The problem with Cimmerian is, that ponies in general aren't much of a challenge to any dragon, and this particular dragon is feared among his own species. It would take a really clever plan and/or something incredibly powerful to take Cimmerian out. Ponies are just not up to the task.
I think the protagonists would not be so keen to have a villain hanging over their heads for the rest of their lives and waiting for him to die off, when he would probably outlive them AND their descendants many times remove before he expires. They certainly would not give themselves the benefit of the doubt that the villain would not try attacking them again.
In most stories, if they have villains like that, the more intelligent protagonists would had choose to take the initiative and find ways to take out the enemy first.
But I concede to your point. Ponies aren't the kind of characters who would be willing to take out villains that they can't befriend, nor do they look like the kind of characters capable of taking out an evil dragon...