We do know for a fact that Amaurot's proposed sacrifices had souls. But let's deconstruct this a bit further. The reverse process is personification, in which we attribute human characteristics to non-human species. It makes us more sympathetic to them, because there's that common ground that lets us see ourselves in them.

Let's take a closer look at these 'plants' that you just sneered at. Etheirys's seedkin are particularly interesting in that it becomes much harder to draw that hard distinction between humanlike and not. You have the tall humanoid Leshy that walk the earth, and you have the Yukinko who make their own clothes out of grass in cold weather, a behavior known as 'wintering'. And let's not forget the Sabotenders and their dances. Any one of them could potentially be a 'tribe' with its own culture, friendships, love, and way of life (imagine a city of morbols). What if they too had souls? Would you be equally willing to murder them just for the sake of a dead Amaurotine? If so, your actions would be no better than Emet's.