Granted, this was a pretty extreme example, but we've all seen this sort of thing happen before. In fact, I'm sorry to say, I have—on occasion—gotten sucked into something similar. Usually when it happens, I catch myself, and immediately try to soften my response. I hope I don't argue with students as much as I used to.
The whole idea of presenting information to adult learners is to give them something they need, and something they realize they need, and then to make them think about it. One of the best ways to do this is to get an interaction going between your students and yourself, or better still, amongst your students themselves, with you as the moderator.
There is an art to this. The idea is to pose a question that isn't too threatening, at least at the beginning. Something you're pretty sure most of the people in the room would agree on. In that vein, the old speaker's rule to avoid politics, religion, and sex is still as valid as ever.
Pick something that is hard to argue with: the need for enhanced officer safety, the shortage of dollars in the budget for whatever, or some other universal topic. Posing questions related to these types of topics will get people talking, and will make them feel comfortable in doing so because they will also know that most people in the room are on the same wavelength.
As your students start responding to your comments or questions, instead of answering them directly, ask another student in the room to respond. The idea here is to break the ice and to get them talking, hopefully with each other.