What's the difference between "to kill" and "to kill away"?
I believe there is a difference.
Imaginary scene: (Don't do this in real life )
Someone is in your way, let's say a security guy.
You kill him away because he is in your way - something like a shield.
After you kill him, you can continue your walk.
I hope that does not sound too violent.
P.S. Not a teacher, and as said, it's more a guess...
They might say, '... kill away the weeds from the border of the garden' or "This product kills away the mold in your bathroom".
This is because, by killing mold, it seems to go away (whereas the guard doesn't). Even so, most people wouldn't say it.
It could also mean "Go ahead and kill", as when the leader of an elephant safari hands out the rifles and says "Kill away!"
I agree; it sounds pointlessly violent.