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”.
And how, pray tell, is a “real” firearm going to help defend against the most technologically advanced army in the world, who can take you out from half a planet away without you ever seeing anyone until you are ginsu-bladed in half (reference: https://www.pcmag.com/news/us-used-drone-carrying-knife-missile-to-kill-al-qaeda-chief)?
We don’t need more firearms in personal hands when it comes to our national defence, we need alliances, a better standing army, and probably some form of nuclear deterrence. Thinking you’ll make any meaningful difference with your personal firearm and a couple of hours on the gun range is the exact same magical thinking 2FA nuts keep using while the US government takes their rights and freedoms away.