Failing to take measures to prevent a breach of the peace that they had been warned about would be bad publicity, at best.
But I'm not inclined to take on cops and robbers at the same time. Like a circular gunfight, except with me in the middle.
If you did tell the police that you were going to take the law into your own hands, what would happen?
2) you'd be arrasted, or at the very least told that if you do that, you will be in trouble.
Not necessarily. I think just carrying a weapon might make the other party more likely to cooperate. Even if you don't take it out.
But I bet that if you did that police would show up as there is an implied threat of violence. I mean, clearly it applies even in OP's case, since police did show up - I'm just surprised he didn't get in trouble for basically using the police as his personal bodyguards by implying things will get violent.
It's hard to put into words, but as a Texan, I can see how police would react exactly like they did. No guarantees, especially for minorities, it could go very badly, but the whole situation isn't surprising to me at all as a Texas native.