Why is breeding cattle for food unethical? Because we have to kill it? You have to kill a plant in order to eat it as well, how is this any different?
I just had a huge debate on /r/vegan to see what vegans thought about this because I was genuinely interested. I'm a meat eater, and I see killing plants and animals as equal, and personally I don't care what people eat. What they told me was that it all comes down to their definition of "sentient beings" and how they think killing "sentient beings" is morally wrong.
By definition, Sentient means: "able to perceive or feel things." - which by definition for me, includes plants because there have been studies that have shown that plants can feel and perceive things that are happening to their own self and other plants within their environment, use memory recall, yet they just show that perception with chemical releases or slight movements. The vegans told me that they see sentience as a "human-like" trait, and they use higher-levels of consciousness to defend their position. For them, that is the difference that plays a big role in why they think plants are not sentient but animals and humans are - which I thought was bonkers because they are going against the definition of sentient.
I also believe that they use this "human-like definition" because of the way that plants express themselves (Using chemicals) in relation to how animals and humans express themselves (vocal expression/facial recognition). They think that eating plants is somehow okay because plants can't talk to show negative/positive emotions, or use facial recognition to show negative/positive emotions like humans or animals can. I personally thought that was illogical because as I've said, plants use chemicals to show their emotions. I have some references to show how plant defenses work here:
Signal signature and transcriptome changes of Arabidopsis during pathogen and insect attack
An Overview of Plant Defenses against Pathogens and Herbivores
Plants can also do cool things - like hunting for example. Plants can actually select their prey based on fragrances, just as we humans would go "mmmm that smells good, I want to eat it": Video showing how plants can hunt
Here's the study on the memory recall that I was previously talking about too: Experience teaches plants to learn faster and forget slower in environments where it matters. - Journal of Oecologia, Monica Gagliano - PhD in Marine Ecology.