Seems like you're talking about the 6th commandment[0], which is more accurately described as "Thou shalt not murder", rather than "thou shalt not kill".

I like Dennis Prager's commentary on the 10 commandments[1]

[0]https://en.wikipedia.org/wiki/Thou_shalt_not_kill

[1]https://www.youtube.com/watch?v=0RENPaY043o