Why Umbrella, why?

So, my question is, Why did Umbrella have never used Nemesis to kill every single zombie in Raccoon City? As mutch as I know, he was tested once, to kill all S.T.A.R members, to test his "skills". But, never seen to kill any zombies... It is impossible that, umbrella was afraid, that he will be bit by a zombie, becouse:
1) He was bitten allready in first movie, when he was still alive
2) He is somewhat of a Robot, (I gues, he can't feel pain)
3) He has a blody Gatling Machinegun and rocket launcher.

P.S Also, just wanted to know, where did he disapear after 2nd movie of Resident Evil?

Regards, Vairis.