If you are talking about the parent comment the joke is that because the dad is a cop and they usually beat on black people, "it wasn't dark enough to beat it to death" is the point. I know it's not well explained but hopefully you got the gist of it.

If you are talking about OP's post then the joke is that a while ago there was a school shooting where the cops refused to enter a school to do their job when they clearly took their own kids out first there's more to this but the relevant part to the joke is that the kids dad is a cop so he refuses to go inside the school.