Ever heard of canine distemper? Probably not, but it's a somewhat widespread disease among domestic dogs. It's not terribly common in the wild, but it does happen there, too. Considering that canines have existed for millions of years, it has probably destroyed the lives of many.
It also affects felines. Here's a lion having a seizure as a result of it.
Imagine being in this position: You've been on Earth for ten years, stealing corpses from females of your species who are actually fast enough to kill their own prey, harboring absolutely no regard for said females' well-being. You spend ninety-five percent of your existence extremely pissed at everything that gets in your way, including your own children, who you routinely swat in the face mercilessly while trying to nap -- because, hey, they're annoying, right? On the rare occasion that you actually get some action from one of the female lions, she sinks her humongous claws into your thigh mere seconds after copulation has completed, because she's moody and fickle.
To some, not having to work to get a meal might sound like the ultimate existence, but if you're agitated the entire time, where's the fun?
Angry and in bully mode from birth to death or not, you're still the king of the jungle, though, right? You'll probably die at eighteen of old age, sleeping peacefully under a tree as your heart gives out. You caused dozens upon dozens of organisms to suffer horribly, but someone had to come out on top; that's the whole point of the struggle of nature.
Except that you didn't really win anything, and you're not going to die peacefully under that tree. You're probably going to die in a bloody fight to the death against rival males -- and, surviving five or six of those in your lifetime, maybe you'll die of starvation after being deposed by a young stud, who will subsequently proceed to hold your family hostage and murder your children.
Or maybe you'll have upwards of five muscle-wrecking seizures in a day, feeling completely out of control and utterly exhausted as some invisible force slowly annihilates you, capitalizing on your ignorance and lack of any capability to fight back.
The real kings of the jungle are all viruses and bacteria.