Is our dominion over animals unethical?
It is the latter, ethical standards cannot be applied to creatures/things which cannot comprehend ethics. There may be other, practical reasons to treat these creatures or things well, or wny number of ways, but ethically isnt one of them in my view.
Your counter-examples dont hold much water, i dont think how we treat dead bodies matters ethically, and someone who is so compromised mentally (psychopaths, severly mentally disabled etc) they cannot understand ethics likewise do not matter ethically. We are free to make simple practical decisions in those cases. A bullet in the head for the psycho seems fine to me. For the severly mentally disabled, I realize that the sentimental attachments of loved ones are real and important so am happy for those loved ones to decide on the treatment etc, but I wouldnt consider it an ethical matter except where practical matters interfere with the ethical concerns of actual moral agents. Similarly, I think a pet is in the realm of sentimental attachment and emotions and should be considered in a practical way or according to the preferences of the pets owner. I still do not think it makes sense to apply ethics to creatures not capable of them.
So I think what you are left with only one counter argument here, that under my view it would be ok to torture the psychopathic, the severly retarded, or pets. I would answer that absent a practical reason to torture/harm the subject, the moral agent could only have immoral reasons for doing so. This is where I would apply the sentiments you expressed above about amoral creatures entering human moral spheres.