Aw, sadly lots of toddlers have accidentally deceased a small animal. It's a hard lesson on careful handling that most remember, even when they forget how they learned. 

My nephew accidentally killed a small kitten when he was 3 and a half or so. He was being really careful with it, like a baby, like his mom said. Then before the adults realized what was happening, he tossed it up in the air... like he saw his dad do with babies. But with unfortunate toddler coordination and it went badly.