You clearly haven't engaged with the thought experiment that is the in world ethics of what's actually happened due to the Sundering.
In this setting the statements:
"Ardbert is dead." and "Ardbert still lives." are both true.
Every person who has died in a Rejoining is dead, but they live on literally by becoming a part of the Source's Lifestream. They either go on living by becoming part of a living person, or they join with the rest of whatever soul they are part and parcel to, in the aetherial realm.
In this setting, physical death is not the end, basically. To stop a Rejoining is to say that a temporary life is more important than the infinite soul that propels it, that imbues in it the qualities of life we hold most dear.
Ergo, the only way to actually commit genocide is to destroy souls completely (or destroy everything they could inhabit).
That doesn't mean it's painless. That doesn't mean that it's right. But as it stands it is better to take the actions necessary to heal those still living souls which have been damaged for 12,000 years. To right the great wrong done against them.
And the proof is there. We could not hope to succeed against Emet-selch even with 8/14 soul shards. We needed another shard of our soul to stand up to contest him. To save what was left of a single shard, we required intervention from the future and the equivalent of technically two further Rejoinings. We had to be 9/14 and rely on the Calling of the Exarch, and then had to contest him with the Auracite, all of the Scions with us, and the Lightwarden's aether.
We literally have to accept the fact that we needed the Ascians to succeed, restore our souls, in order to give weight to our will.
All I saw in the story was the Scions bitterly accept this truth, but being forced to ignore it for their own sake. What came after were pleas of emotion.
It is a bitter battle which becomes a weight that the victor may never unencumber themselves from. Such is what it means to wage war against your own kind.