I have a friend who is the opposite. When the chips are down, farts rainbows and sunshine. It's inspiring and annoying at the same time. But I tell her the same thing I tell you. It's not grounded in reality. Look to nature for truth.
People are both evil AND good. Just like nature, we have the power to create and destroy. Nurture and be cruel. That's just reality. And like
@Alaric593 states, it's just your attitude framing things for your own reality only.
Many people who've only seen evil through most of their lives will have a more pessimistic attitude. Conversely many who haven't had too many hardships can have a more sunny outlook. But I've also see people who've endured unthinkable tragedy over and over again, and have taken that as an opportunity to make sure it doesn't happen to others. Sure they can't change the fact that they took the crappy end of the stick. But why not take that opportunity to make it better for others. That's nature. When something is destroyed, something else is created. Like
@Fino says, it's about balance. And human journey is about each person finding their place on the balance scale.
I can have a pretty pessimistic outlook too. Not just because of my past, but as an observer of humanity (and maybe having social anxiety disorder doesn't help). But man, the good stuff, the positive, always swings me the other way like a pendulum. Back and forth. Everyday. But I don't think anyone deserves evil just because some people who act on evil (mostly because that's all they've known their whole lives). I think humanity is still pretty infantile and immature species that's all.