What happened to a slave owner if they killed their slave?

That depends on the time and place the slave owner lived in. Slaves in the antebellum South were legally classified as domestic animals, like cows or sheep. So a slave owner was perfectly free to kill a slave that he owned. The only reason to avoid killing them is that slaves, like other domestic animals, were worth money. It would be foolish to buy a slave only to kill him or her. Nonetheless, if a slave master wished to kill a slave, he had every legal right to do so.