I might have teared up.
It... um... it's a cat.
This is why books have teasers on the back, sometime you don't have time to read a 350 page novel.
Friends die, things go wrong. It's life. Is it horrible? Yes. What you wrote is beautiful, but at the same time you're wallowing in self pity. The cat wasn't. The cat used its last bit of energy to explore and show appreciation for your care. Be glad. This should be something that teaches you that misery gets you nothing. Be like the cat. He taught you something not many humans can.
Sucks you had to watch it die, but eh what're you gonna do.