🐉 What’s Yveltal up to over here?
Yveltal spends a good bit of the modern era resting in hibernation. Back when things were more tumultuous, they often were awoken too early and wreaked death and destruction until they either eventually calmed and went back into hibernation or Xerneas arrived to stop them.
They aren't a malicious deity, just one that was created to bring death and destruction as part of the cycle of balance. So naturally, they have a bit of a temper and dislike being awoken early. (Oh, you want destruction, do you? I'll show YOU--)
Yveltal is a necessary part of the cycle of life. Without them, mortal death would not occur and life would overflow the planet to dangerous levels. Besides overpopulation of larger creatures within a few years, you'd probably end up knee-deep in bacteria within a week. Yuck.