If you ain't at the level of Himura Kenshin or Vash the Stampede against enemies like this, you've got no fucking business trying to fight with some crippling personal handicap like not killing people.
This doppelganger isn't even at Batman level. It goes beyond not killing people to the point where he can't even inflict violence to a meaningful extent... even to save the lives of his friends. In other words he is completely useless.