
I know the feeling - I've never had one pet get another, but I've done it myself. It's been a couple of years now since I accidentally killed my favorite canary, Steve, and I still feel heartbroken about it. She had gotten into the hallway and in her confusion there, I guess she was resting on the floor. Well I didn't know she was there, it was evening and she was the same color as the carpet, I came along... and she didn't get out of the way.
At the time, none of my birds normally went to the floor, so I didn't even think to look. Now I have the cockatiels, budgies, dove who all love to forage on the ground and I am much more attentive to know where they're at.