While I agree with your main point (which to me sounds like "don't let your cats outside as they're an invasive species and will do serious damage to the local ecosystem"), suggesting to kill them on sight is a bit harsh and won't make you many allies.

We have much more humane options to capture, return, and if needed rehouse cats. Remember that these objectively invasive animals are also people's companions, and that some folks simply don't know better than the old belief that "cats should roam".