You actually enhance my point for me.
This was not secret footage taken by somebody at an execution. This was not downloaded from a phone camera and smuggled onto the internet.
This was a NEWSREEL back then. Something that was shown in movie theaters before the film, along with cartoons. It never even occurred to me that a warning might be necessary.
It's a serious point of debate, with no easy answers, whether we were too hardened back then, or too soft now.