Vegetarianism in the Ancient World

Michael Gilleland has an excerpt from Seneca describing his conversion to vegetarianism and subsequent lapse.

Note this part:

Sextius believed that man had enough sustenance without resorting to blood, and that a habit of cruelty is formed whenever butchery is practised for pleasure.

It’s interesting that this remains, in all essentials, the core of the contemporary case for vegetarianism. Since human beings don’t need to eat meat to maintain health, can we justify killing animals (“resorting to blood”) for mere pleasure?