"Also, I'm opposed to assisted suicide as well." - well at least you are consistent.
"Except most pets euthanized aren't ..." No, but that's not really relevant. Pets who are owned, homed, and loved, are only euthanized when they are suffering.
What we do to pets who are not owned and who have no natural habitat to survive in (unlike deer who are culled because of over population, and us having wiped out their natural predators) - we have become the ones to control and dominate the environment. Thus putting them out of their misery, because a pound can't afford to feed them is indeed to avoid suffering.
Is your arguement that we should treat animals the same as humans, and let them choose suicide if they can't afford to feed themselves? Or that we should make sure suicide is done in inhumane ways??
" the futility of religion and the way religions and their schisms hold back humanity"
No, religion here is merely a symptom. You could use any other political division to divide people, invented or otherwise. Eg: 'tribal' rivalries in Rwanda, linguistic/ethnic divides, political ideologies in the cold war.
People will label their rivals in any way that they can in order to exert power over them. Or use labels that already exist as a way to rally people against the other. Any possible identifier... Yellow stars for the Jews, Pink stars for the Gays, Skin colour, whatever can be used by people in power to exert that power over others.
Religion is just an example, not the cause.