Just a quick disclaimer: I'm vegan, which I'm well-aware might make some people automatically hate me (I've been around the block a few times... that said, it was mostly on Instagram, which is just a cesspool most of the time, anyway) but I'm not looking for an argument or trying to attack anyone or to tell anyone in particular what to do, with anything that I say. If I ever say the word "should", I'm just using it in the context of my personal point of view, which I've massively simplified here.
TLDR: I'm just here to share my perspective. :)
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I think we all inherently have that biological drive to survive and keep our species going. The difference between plants and non-human animals, though, is the presence of pain receptors and a central nervous system. Plants can react to stimuli (e.g. a Venus fly trap), but I haven't seen any peer-reviewed reports conclusively stating that plants have pain receptors/CNS, and thus are able to feel pain. Non-human animals, on the other hand, have both of these things. I think the question is not so much one of intelligence, but rather, if they are sentient and capable of suffering – I think most of us, at the very least, would agree that cutting a carrot isn't the same thing as cutting an animal's throat. Even if plants do feel pain, though, it would still make more sense to adopt a plant-based diet, just mathematically speaking, as animals typically used for food consume way more plants than we would if we just ate the plants directly. For those who say, "Well, the animal is already dead, anyway,"... this is true, but it's only because people continue to support it with their dollar. It's just supply and demand – if you pay for something to happen, then it will continue to happen. If you don't, then it will steadily go down, as the statistics have unambiguously shown, in even just the past ten years or so. You know what I mean?
We humans love to think that we're the best things that have ever happened to this planet, that we're number one, etc, and that because we objectively have more power over other living beings, that we have the right to do whatever we want to other sentient beings because we're more powerful (it's a very "might makes right" line of logic). But I think that with a great amount of power also comes with a lot of responsibility – among them, protecting those who are more vulnerable, rather than exploiting them. Humans have just done so much damage to the world, and as a species, we're arrogant as hell on top of that. This is one of the (many) reasons why I don't want to live anymore; I simply can't watch us continue to harm other sentient beings (human or not) and recklessly destroy this planet anymore. It's depressing and makes me feel completely hopeless.
I can't really say what killing oneself would do as it relates to your question (I recently took a muscle relaxant so I'm a bit loopy at the moment, but I might come back to that after giving it a little more thought!), but I do think that we should aim to minimize the harm that we contribute to in the rest of the world as much as much as we can, while we're still alive. Yeah, this world is absolute shit, and it's unfortunately not possible to live completely free of harm, but I think that what matters is that we do the best we can.
Some of the things I've said might piss some people off, which is absolutely not my intention, and I'm genuinely sorry if that's what happens. (I guess this could be said for just about any answer on this forum, though... humans are interesting, I'll give us that.

) Again, I'm just sharing my point of view and answering the questions that OP posted, as simplified as I possibly can. :)
This is a good question/topic, OP, and I've enjoyed reading the answers so far.