Under Kane and Finger, but then the Comic Code mandated that he couldn't kill anymore. Also, I would add the classic Ten Nights Of The Beast storyline. Batman locked the KG Beast in a room and left him to die. At the time, it was pretty clear that he more-or-less killed him.
If memory serves, and it may not...
The comic code came along in the mid-50s, over a decade after Batman stopped killing people.
Batman not killing was more to separate him from The Shadow, which he pretty heavily "borrowed" from.