You're right that the issues are more nuanced - I just gave my bottom line analysis based on the rules of evidence (roughly paraphrased):
Rule 402: Relevant evidence (i.e., probative of a matter at issue) is admissible;
Rule 403: Even relevant evidence can be excluded if the danger of unfair prejudice outweighs the probative value of the evidence;
Rule 404: Character evidence is not admissible except under certain circumstances in criminal cases; evidence of other crimes or wrongs are not admissible to prove the defendant did the same on this occasion, BUT can be admissible for other purposes, such as to prove intent, motive, state of mind, etc.
So synthesizing the rules and applying them to the evidence, I don't see how the video of KR the few nights before shows specific state of mind the night of the incident. To me, it's evidence of a teenage kid talking **** to his friend, not evidence of specific intent. Hell, if I saw certain politicians in public, I might muse aloud about wishing I had my AR. Point is, it's not a comment that deserves to be taken seriously without much more. And any probative value without more is outweighed by Rule 403 considerations.
With respect to Rosenbaum's past, a history of violent or volatile behavior could be relevant to whether he was the aggressor in this instance. A history of sexually abusing boys, though? I don't see how that's relevant at all to whether KR thought he was a serious threat of bodily harm or death at the time this occurred.
