Originally Posted by
Enkidoh
The Archons were spared and arrested by the Garleans because of their potential intelligence and information they might possess that would be useful to the Garleans, specifically concerning the Echo and Primals (you see this later in a scene in Castrum Centri where Livia is seen torturing Minfilia for information about the Echo) - the main fatalities in the Waking Sands were just random adventurers and Ul'dahan Immortal Flames guards and hence 'cannon fodder'. Tataru being the doorgirl and secretary also would have knowledge of the comings and goings of the organisation, and especially potential whereabouts of the actual target of the raid (who wasn't even there) - you, the adventurer who became the Warrior of Light - so she was arrested too.
Remember, Lahabrea had possessed Thancred by this stage and would have had access to his memories and knowledge - he would have known exaxctly who would have been the most important for information to the Garleans, and simply instructed Livia on who to seek out. Also the Garleans' long standing hatred for the beastmen meant Livia's especially brutal killing of Noraxia was hardly unexpected - the poor little sylph was not even a sentient being in Livia's eyes, just yet another monster to be put down.
The attack on the Waking Sands was to gain information about the Echo, specifically it's protection from Primal tempering, and in particular capture one particularly potent Echo user that had started making a name for themself as the 'Eikon Slayer', but failing to apprehend you, the Sharlayan Archons along with the Scions' leader and her secretary would naturally be the most useful, everyone else in there is simply expendable (and thus slaughtered). Also, Livia herself kills one of her own troops for being too bloodthirsty (as I recall the soldier was in such a frenzy he was basically butchering an already dead corpse with a spear), so it was zigzagged as to the Garleans unnecessarily brutal response on the place.
Either way, the attack on the Waking Sands somewhat failed in it's objectives - it was to capture the future Warrior of Light in order to gain some insight into how the Echo protects against tempering, but Livia utterly failed in that objective, and so simply arrested the next best thing - Minfilia, Tataru and the Archons who she hoped at least would field some useful info. Everyone else sadly in the Waking Sands at the time was superflorous, and thus condemmed to death for being witnesses to the attack.
If Arenvald had stayed and guarded Minfilia and Noraxia, he would have almost certainly been killed by Livia anyway, a noble death is still a death that would have been totally avoidable.