"but we still treated animals like shit for many many centuries!"

Actually we still do, probably now more than ever. The killing portion is more humane, but the way they are treated the rest of their lives is often appalling.

Chickens are cooped up in, well, coops that are so small they fight with each other causing serious damage with their beaks, so industrial farms use heated blades and cut off the beaks to prevent this. Cows are given terrible horomones to make them give more milk (only where officials are stupid enough to allow this, USA) that cause horrible side effects. Pigs are put in stalls not much bigger than themselves, and basically can't move their entire lives.

I feel a little less guilty by buying meat directly from farmers that free range their non-GM, horomone free animals.