I've been lucky enough to never see an animal die. However, I have had a pet murdered by a human. I was living in an apartment above a garage in northern Louisiana. One of the tennants of the duplex on the property bought the place.
About a week before my lease was up, he or one of his parents shot my cat in the face, and left her on my porch. I was utterly devastated. I had no way to prove anything, even though a gun had clearly been discharged in a populated area, and my cat was clearly dead.
A few days later, they illegally evicted me by cleaning out my apartment and taking whatever they wanted, under the guise of "assuming I'd moved out early." When I called the police, they hit on me, and told me to make a list of everything that had been stolen and to contact them if they didn't give it back. And they did nothing about my dead cat.
The entire situation was completely heartbreaking, and I still find myself missing my old friend Daria regularly. It's amazing how callous and selfish human beings can be.