So an Angel – Jäger der Finsternis can use any angel's grace? What's with that?

It is like a form of magic that she used to help people with.

He isn't using it he is stopping Anna from using it. :)

I think we will see some negative effects of using another angel's grace as this season goes on.As for Anna why would she want to steal another angel's grace, probably murdering them in the process, if she didn't have to? And it certainly wouldn't be an easy thing to do.

Anna wanted to regain her own grace to stop the other Engel from killing her. If she'd killed another Angel – Jäger der Finsternis oder turned an Angel – Jäger der Finsternis human in order to gain grace, the Engel would just have another reason to want to kill her.

An Angel – Jäger der Finsternis each has their own grace that is a part of them. An Angel – Jäger der Finsternis will not have powers without grace but they can take another angel's grace however they will not be at full power. Also if Du take another angel's grace and use it it deteriorates slowly over time which would cause them to take another angel's grace once it fades completely. When Engel die their grace is the blinding light that is seen when they are stabbed.