It doesn't have to be said. All the announcement does is tell law abiding customers that some of the people they may share a train with might be abusive. It's not required to be said and they should stop saying it.
The problem, sadly, is that it does need to be said. Yes, it will probably not deter the people who are abusive, but it will take away their lazy defence of "well, nobody told me I couldn't be abusive".
And then there are conductors who announce "please get your tickets ready, a full ticket inspect will now take place" and over an hour later at terminus no ticket inspection has taken place. ...
Possibly because they became bogged down in a problem/enquiry and didn't make it to where you were sitting.
The conductor who switched the lights off and went "woooo" needs to be walked off site asap. Worst of the worst. Unprofessional at very, very best, disgraceful conduct Here to get us from A to B and your playing stupid games. That is me being nice about that incident, and thats not the first time he's been a ..... with his onboard comments.
It's against the rules to run in passenger service with no lighting in a tunnel/darkness...if it isn't possible to take a vehicle with failed lights out of service the guard would be expected to stand in it with a torch... so no, they shouldn't be deliberately turning the lights off, even momentarily.
However it's a rebrief and warning not to do it again, not an instant default to sacking!!
I'm guessing you did actually get from a to b, despite the conductor's frivolity?