Yeah, it's the nature of the Beast that both of them are scum and Thedas is NOT FAIR.
I like that historical authenticity.
It's a shame that neither one of them thought arresting a noble because he killed someone was a possibility. Even Halamshiral's lord (Pierre?) had to let the elves take the law into their own hands to see justice done. Would having judgement applied equally regardless of social status have sent the wrong message? It would probably be tricky and impractical to implement, but the way things are lets nobles know that their faction needs them more than they need their faction.
See but neither of you are seeing it from their perspective or the world they live in. Its an issue that arises when using modern morality on a world it doesn't exist in.
The idea of arresting a noble for crimes against a peasant wouldn't have worked in midevil times in our world let alone in Thedas. Gaspard is a chevalier, he can kill any peasant-elf or human- without having to answer for it at all. He sees elves as inferior and literally blames the state that Halamshiral was in on the lord being to easy on the elves and not crushing the revolt harshly.
Celene sees the political reality of what she can do and can't do. She is already pushing for equal rights for elven merchants and allowing them to own and sell goods in the merchant quarter. She has also forced the university to admit elves, and modernized their teaching to educate the future generation of nobles into the idea that elves and humans are equal. She is making strides, but she also knows good and well that Gaspard is plotting against her and if she hauls up a noble and publicly punishes him for crimes against a lowly gutter rat elf(the ways the majority of nobles would view him, not me), then the majority of them will go over to Gaspard and she will have lost the Civil War before it begins. As empress she has great powers, but she can only maintain them as long as she has the support of the majority of nobles. If she pisses off enough of them, Gaspard and his forces can basically walk into the capital without her having enough support to repulse them.