Well, your friend and Zack Snyder are both idiots.
The original Batman did carry a gun for about a year, until the creators of the character worked out his backstory and what it meant for the character's use of violence, and what his limits would be.
Since then, for around 80 years, Batman in the comics did not kill. So, to say that one year at the start counts for more than 80 years since is just stupid.
Zack Snyder is a great directors of dark stories, but he's not very willing to learn specifics about existing characters. He did a very good job with Watchmen, but that was a dark story, in a dark universe, with characters who were all dark, just to different degrees. And, there wasn't 75 years of backstory that most comics readers know very well with the Watchmen characters and universe.
Thus, the movies are more about the director than the source material, and the DC movies have not been well handled from that point of view.