The treatment of death is always going to vary by culture and religion. I also don't believe the LTW quests somehow nullify the gross amount of senseless killing that occurs in the world either, which more often than not doesn't result in the procurement of crafting materials. Not to mention that living creatures don't give a toss if you're thankful to them, you're still taking their life against their will. I could say I'm thankful for the shards dying so that my WoL can become more whole, does that make rejoinings okay?

It seems like people are looking for reasons to condemn the Ancients so they can feel self-assured they deserved their fate or at least convince others they did. Even if they were unquestionably bad I would still struggle to make that judgment. The fact that they weren't and you have to nitpick to find (subjective) reasons they weren't good speaks for itself. There's no evidence there was war, murder, sex trafficking, etc. (all things that exist in the sundered world) in their society, which by all appearances was peaceful and prosperous. So, instead, we're going to buckle down on the fact that they had a belief system individuals (certainly not consensus) don't agree with and weren't vegan.