Batman was known for killing, just usually not with guns. This isn't anything out of the ordinary for the original Batman.
He also kills in the original Batman (outside of the already mentioned Joker and Axis chemicals) - he fires miniguns and rockets into crowds of the Joker's thugs from the Batwing while he's breaking up the parade and he clearly kills several.
Batman in the comics used to kill people all the time. Burton's Batman is based on the initial incarnation of the character, hence him killing several people over the course of the films, including The Joker, that goon he drops from the belfry, and everyone in Axis Chemicals.
He did kill in batman returns but this is the director Tim Burtons doing...Batman (the comic) is not a killer and does not kill, Batman has had different directors so you will see minor (mor major) discrepencies