In this case, one could argue as follows:
Nina disappears, and one or more friends help her get to Russia incognito (go through Mexico, perhaps).
Children go into custody of her parents who take them back to Russia.
Meanwhile, Hans is the first suspect, as is always the case in murder cases where there is a spouse or former spouse with some plausible motive and no good alibi...he is known to be paranoid by anyone that knows him, so begins to behave erratically when police begin to pay attention to him.
Perhaps someone (maybe Nina, maybe the serial killer friend, maybe someone else) had sprinkled a little blood on the front seat of his car (and in his house and the sleeping bag). He panics when he sees it and rips the seat out and disposes of it. Since he didn't commit the crime in this scenario he wouldn't have even known about the blood in the sleeping bag or the house.
Since producing some blood is pretty trivial, it means that anyone with the means and willingness to disappear could frame anyone for murder.
There is a reason why murder cases without a body or murder weapon pretty rarely result in conviction. And, I suspect, had Hans taken his lawyer's advice and not taken the stand, he would have probably had a much better chance, even with the pretty strong circumstantial evidence.
I'm not saying Hans didn't do it...I don't know. But I know that circumstantial evidence should be viewed with suspicion, when 25 years of someones life is on the line. The jury system is intended to insure that no innocent man is ever convicted, and this principle is even more important than every guilty man being punished.