So you can not torture a plant or make it suffer.
Don't tell that to some of the plants I've seen in my office. I often end up watering them because no one else does.
That’s why it would feel more painful to watch someone rip the legs off a spider than watch someone rip the legs off a cat or horse or chimpanzee.
That sounds backwards. I'd hate to see either --but I'd definitely hate to hear a cat/horse/chimp get their legs torn off, much less watch such a grotesque action.
They have a much smaller capacity to suffer.
I'm no angel, but I was fascinated how a daddy-longleg spider's leg would keep moving after it was ripped from their body. I did it a couple times before I could plainly see/feel they were suffering. Really, I can't judge how much a creature suffers more than another.
So there’s a spectrum of animals ordered by how self-aware they are and how complex their thinking is: spiders, fish, chickens, ravens, octopus, cats, dogs, pigs, cows, horses, dolphins, gorillas, chimpanzees, humans... roughly something like that.
I can't remember the German band's name, but they had a nice take on that order: GermWormFishAmphibianReptileMammalManOblivion
Everyone draws a line on the spectrum, whether consciously or unconsciously, what they are comfortable with. Some people are fine eating fish and chicken, but not pigs and cows. Other people are fine eating pigs and cows, but not chimpanzees, who are almost human. Some people are even fine eating chimpanzees and feel no empathy when they shout and panic.
A friend and I were talking about how interesting it is that we love our dogs in the western world, but in parts of Asia it's completely acceptable to eat dogs. The USA over-indulges on cow meat, but in India it's a sacred offense to do so. There's no way I'm eating cheese with a bunch of live worms crawling around in it. What people find acceptable to eat is culturally bound.