Why do the Androids want to kill Goku?

Gero’s plan was for Cell to get an infinite energy source, and receive monumental power jumps in the process. 17 and 18 were more or less rebellious to Gero’s control, so he used his futile creations as batteries for a perfect being. Cell was not only programmed to go after 17 & 18, he still wanted to kill Goku.

Do the Androids kill anyone?

The androids in Trunks timeline were different, and they definitely killed people. They were displayed as much more sadistic and evil, however according to Trunks, the Androids in the main timeline are much stronger.