Things happen. That's life. Sometimes things happen that we don't like, things that are uncomfortable or sad or just regretable.
A few nights ago I watched a cat die. It got run over, and all I can say is that I froze. It took all of thirty seconds, but for that time I didn't know what to do. I was scared to race out there and help it, uncertain, and at the same time numb.
But it occurred to me that I kill my characters all the time. I don't even think about it. If the storyline goes that way, they die. It's usually for a good reason (in the story line sense).
In real life I avoid pain, conflict, etc, but in my stories I don't hesitate to use these things.
In the words of someone, somewhere, I put my characters up in a tree and throw rocks at them. Preferably there's an alligator waiting at the bottom of the tree in case they fall.
While I would likely never have reason to kill a cat in my writing(somehow the animals always escape unscathed) I have those emotions in my head now.
Chances are good that at some time one of my characters will watch someone die and those emotions will be there.
Those sad, uncomfortable or regretable things that happen in life somehow make it into my writing, and my characters live the emotions that I was too numb to feel at the time.