This was a great movie for three reasons. First, perfect pacing. It starts off pretty interesting, builds up in the second act by presenting questions, and in the third act has you dying to know what's going to happen next. The ending scene was done really well, although I can think of an outcome I would have liked way more.
The pacing of this movie was done almost as well as Das Experiment, confirming my prejudice that Germans are just awesome at planning shit out.
Second reason this movie was great, it blurred the lines of morality. Good vs evil isn't just overused, it's unrealistic. In the real world, there are no good or bad people, but there are plenty of people trying to be "good." Antikoerper presents a conflicted main character who comes off as very human, struggling to be a good person while his "bad" animal instincts are being awakened. The blurring could have gone further, it could have gone into how good/evil is purely subjective, but for what it did, it did it very well.
And lastly, there's the killer. I loved this guy. He's one of those bad guys who just loves being bad. He mindfucks the main character into questioning everything he knows, sets up an awesome plot, and has the mannerisms of both a sadistic bastard and an evil genius. And yes, the awful german language helps his intensity.
If I had to nitpick, some of the decisions the main guy makes are a bit out there. Not so out there that you'll be like, WTF?, but just out there enough to make you think it's kind of unrealistic. Still, it earns the four stars I rated it.