As I have said before, and which I am not sure how you still have not understood or comprehended this, accidentally killing someone IS NOT MURDER, therefore, the killer SHOULD NOT BE EXECUTED!
This passage SPECIFICALLY deals with this sort of scenario.
“When the Lord your God has cut off the nations whose land the Lord your God is giving you, and you dispossess them and dwell in their cities and in their houses,you shall separate three cities for yourself in the midst of your land which the Lord your God is giving you to possess.You shall prepare roads for yourself, and divide into three parts the territory of your land which the Lord your God is giving you to inherit, that any manslayer may flee there. - Deuteronomy 19:1-3
http://www.biblegateway.com/passage/?search=Deuteronomy19:1-3&version=NKJV
The evidence (remember, two or three witnesses) would show that he is not guilty, but that it was an accident.
Answered above and prior to this post, here in this thread and in other threads.