We all draw lines between what we do and don't give moral consideration to, and these aren't necessarily purely subjective. There's a reason why almost no one cares if you slice open a grapefruit compared to slicing open a living person's head: we believe sentience and consciousness to be some significant, valued attribute, and at least to some degree non-illusory.

The current scientific understanding is that most animals are so much more sentient and conscious than all plants that it makes sense to give moral consideration up to some threshold lying between the two. (With unicellular organisms even lower.) There are some animals (e.g. jellyfish) that as far as we know appear to reside below the plant threshold, in which case I'd consider it okay to kill and eat those animals.