That honestly seems more akin to "Ferelden doesn't like slavery" along the same lines as "orlais doesn't like slavery". Remember, even though Orlais is the nation that has elven purges, Ferelden ALSO is noted to have these and Isabela herself mentions in DAII that it is the Orlesian navy that pirates/slavers fear. Both Orlais and
It says nothing of what they think of elves specifically. IIRC, wasn't there even a Chantry mother at the wedding who tried to stop Vaughn and he still took the elves away? There's also the fact that technically, the city elf was going to the fallows NOT because she killed a noble (Vaughn is still alive in many scenarios) but the killing of the guards even though Vaughn had kidnapped all those women.
Orlais might be hunting pirates, but it's also repeatedly implied or even explicitly stated that that the nobles do keep slaves and the practice is hardly uncommon even if they do care for appearances not to call them slaves. I think part of Celene's reforms was an attempt at enforcing anti-slavery laws to any extent. And that is what they do on their own yard, since we're told that Orlais after taking control of Ferelden declared Fereldan elves to be property and started selling them; it's implied that it was done officially and on large scale, though I must say I don't buy it entirely (unreliable narrators of codex entries and things like that). Still, I believe the practice was in place and left completely unchecked as a way for nobles to get some extra gold out of the whole invasion mess.
As for the guilt of City Elf, I wonder what would happen were Vaughan killed on the spot, there in Alienage. When it all happened in his residence, though, that can change a lot - a couple elves going on a killing spree in arl's mansion doesn't look well. Note, however, that even if both you and Soris keep silent and that b*tch rats you out, Soris is not judged and hanged - he actually ends up in Howe's little private dungeon. And, as we know, you don't exactly need to be guilty of anything to end up there. Seeing as city guards most likely are directly responsible before the Arl of Denerim (then Vaughan's father), I woldn't put it above lord Kendells to throw the "guilty" elf into dungeon wondering what to do with this mess to not send the message that you can just barge into Arl's estate but also not to have a blatantly unjust execution on your hands (especially as the mother, while unwilling to really stand up to Vaughan, could've testified or something...). There could also be the element of vengeance if Vaughan died. Either way, however, Soris apparently lives and serious repercussions for Alienage only start with Howe... who most likely needed a pretext, not a reason.
Basically, I'm willing to believe that were the justice (or Fereldan perception of justice, more specifically) served, Warden and Soris could very well have gotten away with killing Arl's son. Of course, when your opponent in court is the Arl, your chances of getting free trial are greatly reduced, but in Orlais it would likley be the law that kills the "murderous elf" in similar situation, not any abuse of law.