To clear up terminology, I think genocide is the large scale murder of those who belong to a certain group with the expressed intent of murdering them. E.g. Adolf Hitler expressed his intent to eradicate all Jews and murdered them on a large scale, so that's genocide. But: cities might be sacked in Medieval warfare and murder on a large scale would take place, but since the expressed goal wasn't to murder everyone, this doesn't count as genocide (in my book).

With that said: when did genocide become recognized as a distinct act? Is there concensus among historians on what constitutes the first genocide? The earliest genocide I can think of coincides with the advent of modern war, namely the Armenian genocide. Is this in fact the first genocide recognized as such, and if so, is it a coincidence it happened when it did, namely when modern warfare made killing people easier?