Well, Casey might have killed in revenge, you are missing the point of the episode.

While Nick is a Grimm, he is morally conflicted with killing Wesen. He is not prejudice towards the Wesen.

But Andre has destroyed people's eye sight on purpose, Nick was close to lose his as well.

@Matipereira - I'm not excusing Nick being torn between being a Grimm and a cop, The cop part of him should overrule the Grimm portion and he should arrest Casey.

But Hank? Hank has no such legacy or background or even a moral conflict. He's human, Casey committed a crime, and as such pay the price.

Great question!

I actually liked Hank didn't take her to jail because i think people who kill for no reason deserve to be killed

Well to each his own.

Casey killing Andre could be seen as self defense, she feared for her life, he'd been hurting people, and he came at her in what she could assume was an attempt to hurt her. They might not have arrested her, but sent the case to the DA to decide if they were going to press charges. Even Sean could have said not to arrest her. Not everyone who commits man slaughter (which this would be) is arrested on the spot, it all depends on the circumstances.

Also the show has inconsistencies like this and I just assume whatever happened happened between scene cuts.

If they sent the case to the DA, that would be fine with me. I just don't care for Hank making the decision whether or not to prosecute. If he wanted to go that far, he should have become an attorney.

Sometimes for cops the choice isn't cut and dry. It's all about the legality and if a case can be tried in a court of law. For the fact it was a Grimm case and the subject of why so many people who were not cops were at the scene and Andre was missing an eye would come up in a trial, they might not have been able to do much in court. But I think everything legal about that epi (and most epis) happened behind the scenes.

But here's the thing with that. It wasn't Nick who made the decision. It was Hank.