"It's bad for humans." The scale on which we eat meat is demonstrably unhealthy, wasteful and recent (arising in Western culture in the last hundred years). The way Westerners eat in general is unhealthy and meat is just a part of that, but it's a significant factor.

There was a lot of discussion of nutrition a while ago. The general conclusion is that the literature is too confused to assign any one hypothesis high probability, but was leaning toward Atkins-style low-carb high-protein diets.