The ending is a no win. Everyone is devastated / hurt and no one got what they wanted - not Marc, not Kay, not Brittina, no her parents, no one.
Marc is not free. He has just accepted what he has lost and is tying to move on the only way he knows now, which is to be a better policeman - hence the return to the track / ending.
I think.... a movie like this gives a lot of room for own interpretation. That was exactly the intention of the film makers. About the end they said literally: the end is open, everyone can decide for themselves whether the end is good or bad.
I guess personal life experiences influence the interpretation and I watched the movie from my own point of view.
The ending ( the running scene) has a symbolic meaning: Marc is capable of breathing again. That was my feeling about it but confirmed by the comments. That also was the moment they suggested this could be the beginning for a part 2.
Marc may be alone and devestated but for me he is free to discover himself and rebuild his life again, whatever that may be. Hanno Koffler said in an interview: the greatest personality developments are hidden in life crises. I agree with that.
The scene at the end with Gregor was important. To me it was like: now you know who I am, whether you like it or not. That's a good start.