A plant also has a life. You wouldn't wanna be pulled out of the earth, washed in a sink and thrown into a bowl together with some dressing either...?
And I'm not sure if you can replace a lion's antilope with a lettuce.
I try to avoid farmed fish, which are fed GMO soy and maize. And the Salmon for example which is also fed chemicals to give them color. (I think the meat would be green without).
But wild mackerel and cod from the North Sea,,, mmmm!