Actually, we do definitely know they have no feelings. They don't feel 'pain' like we imagine either.Only they’re not pests, they’re animals you decided to take into your care. And you don’t know that they don’t have feelings, as in capable of feeling pain or discomfort, they react to to touch, they’re obviously capable of feeling. I don’t know, there just something kind of gross and callous about acting like animals are disposable just because they aren’t higher functioning or are considered ‘cute’. I can’t say I’m happy or feel good about having to get rid of living pests in my tanks, I genuinely feel about about it, but if they’re uninvited and hurting something else, I’ll get rid of them. It just seems wrong to me to dispose of them when it generally only takes a tiny bit of effort to find new homes for them (there are already a bunch of people here saying they’ll pay for shipping).
Here, a relatively more 'simple' introduction to the evolution of neurons in animals, and broadly why we can't consider 'primitive' animals to be even close to what we have: https://www.sciencedirect.com/science/article/pii/S0960982216304894