He took an eye
He could half see the sky
When the birds came by and flew
He sat in chair
He was greeted with snare
When the mother dared to look
How could the other
Take from another
When the brother is left the same
To kill for pain
Justice to gain
Killing a murderer leaves the count unchanged
Eye for an eye
Does not mean the other dies
It means to see despite the tears of cry
Green Dharma 
Taking a life is not right in anyway, but showing mercy can also be an insult to the grieved
You’re right Paul, I do see that to be a burden of acceptance and the lesser of two evils for the grieving. They will have to see the murderer live on, and they will have to address the monster, they will have to know it lives away in the cave (symbolically).