I must be missing something. What happened to Cain? I can't find where he died or was killed.

Cain lived to a ripe old age. He was likely killed in a fist fight. It's in Genesis.

Look at chapter 4, read slowly. you'll find it .

There is no mention of Cain dying or getting killed. Maybe he died in the flood.

This answer is not given in the Bible. Only through rabbinical or extrabiblical literature.

Some believe he was accidentally killed by a blind man who was hunting, while others believe a stone house fell upon him.

In terms of the Bible we are left to wonder as he was sent into endless wander.