Life is life - whether in a cat, or dog or man. There is no difference there between a cat or a man. The idea of difference is a human conception for man's own advantage.
'Thou shalt not kill' does not apply to murder of one's own kind only, but to all living beings and this commandment was inscribed in the human breast long before it was proclaimed from Sinai.
The Greeks, with their truly healthy culture, have once and for all justified philosophy simply by having engaged in it, and having engaged in it more fully than any other people.