... so that incest would not lead to the deformations it does now, with us inferior men...
lev. 20:17 doesn't say they die, though. it says they would have been cut off from the israelites (as a group). deu. 27:22 says they would be cursed.
Now, that's what happens when people take the Old Testament and interpret it quite too litterally...
The authors of the Bible were very human and often made mistakes in their writing.