Whatever Ariel's back story ends up being (that is assuming we learn anything about his history), I'm pretty certain that Elspeth won't end up killing him, it seems to go against everything the Ober misfits stand for these days. So, maybe it'll come to pass that they not only send him back to the Land to stand trial, but, if he actually *is* defective they end up going the empathy route and try to heal him. It'd be a solution laced in empathy instead of revenge. Though, as you've said, based on his actions thus far he's almost certainly irredeemable.
I've been wondering more and more about Jes, since re-reading Ober. Rosamunde's story felt more and more like it was a coercive implant, and it wouldn't have been the first time it happened to her. The other thing I wonder about is Jes' apparent killing ability. The interesting thing is that it's because Rosamunde told Elspeth about this ability existing that she realised she could use such an ability when she had to kill Vega (p234, Random House ed):
Then something inside my head crackled violently; a power stirred in me completely unlike any other ability I possessed. All at once, I knew that Rosamunde had spoken the truth: Jes had killed that soldierguard, and I knew how.
Who knows, maybe she would have realised it existed anyway, but it's interesting...
Anyway, there's no reason for Jes to have this killing ability if it only exists in Elspeth's mind to be a tool of the Quest and no other misfits have it (and, there have certainly been times that if it was a power that existed in all misfits minds only to be wakened when there was need that more would have discovered it).
It's more likely that Jes was a coercer, so knocked out the soldierguard rather than killed him and that Rosamunde was coerced into seeing what she told Elspeth. It could have even been that Harald (the one who apparently showed Jes how to use his ability) was an agent for the Herder faction, sent into orphanhomes to find Talents for the Herder's purposes, who planted the entire story in Rosamunde's head when all that had really happened was that Jes and the others he'd discovered were taken away (going with Isobelle's saying that Jes may return, from ages ago).
In the which case, the soldierguard thing probably never happened. But...that wouldn't explain why
Rosamunde would have been coerced to think Jes had killed the soldierguard with his mind, other than to pass that specific information onto Elspeth, which would suggest that Rosamunde, too, was conveying a message from the Agyllians.
I'm so confused :P