“Dearly beloved, avenge not yourselves, but rather give place unto wrath: for it is written, vengence is mine; I will repay, saith the Lord. Romans 12:19”,
Genesis 9:6 is the proper thump.
There are plenty of places in the Bible where death was the penalty for an infraction. I have no problem with the concept.
If we’re talking Cain and Able, of course Cain was wrong because he caused an unprovoked death.
It’s my take that Genesis 9:6 references that type of killing. It’s also addressed in Exodus 20:13.