Why kill someone is a sin

You’ve answered your own question:
When someone murders another, do they do it to eat them? No, they do it to end their lives. That’s what makes it immoral.

I suppose it depends on why you've killed them.
is it to serve justice? Because God says that is his job. are you ending. life you feel is pointless? Again, not your decision to make, you would have cut short someone else's life journey of exploration and decision making.
But you also need to ask yourself, what is sin?
from my understanding sin is anything that separates you from God, but your religion matters here because that redefines what sin, or what is seen as sin, is.

Humans know better than to kill each other

The Law thou shalt not murder is a law given to mankind not to Lions or any other animal..

So people who follow the leading of God endevour to follow that Law..

Oh yes The life duration of a human being should be under the control of The LORD.. If i murder someone i am usurping the leadership of God and thus it is a sin to do so..

And people run over animals all the time 🫤

Lions don't care.. if they sin..

Nor, apparently ,is it a sin when a Religion or Government condones that killing in the name of what ever side they are fighting for.

Lions are carnivores that kill purely to feed.

Men are not.

Human beings were created for a lot of reasons and not eating,sleeping, giving birth and killing around