When Vali asked Lord Rama that why did he attack him from back then Lord Rama explained Vali very elegantly. He said, Vali you have become Aatatai and killing Aatatai(one who killed anybody, capture others property, take away other's wife) is the duty of only Kshatriyas. So there is no sin in killing you from back. Aatatai is like criminals and Kshatriyas are police man.