*I don't know anything about the parent-killing case you're referring to.
To clarify, it's not an actual case. It's a (semi hyperbolic) example given in law schools all the time to explain several things, such as the duty the defense to their client and how sometimes, you can spin inconvenient facts into arguments that may support your claim. To paraphrase one of my professors "When it comes to the law, facts are stubborn but the truth is malleable." So in that case, it is a "fact" that the person murdered both their parents (lets assume the accusation is correct). You can draw multiple "truths" from this, including "defendant has now become an orphan". Is that a reasonable truth? That's up to the jury.
None of this is to say there's no such thing as absolute truth. Only that people's interpretation of it is up for debate.
An expert witness providing testimony should be reliably neutral, it should be exclusively about facts
Expert witnesses are called in to not only explain facts but also give their opinion, through interpretation of facts. For example through analogy, if you go to a doctor and they tell you "you have a 3 cm growth on your left lung" and then nothing else, that's not much to go on. You need their interpretation. Could this be benign? Do they suggest surgery or some other form of test? How worried should you be, in their opinion? Likewise, expert witnesses will almost always explain their reasoning for their conclusions.
I'll give a shit about this when I'm allowed to kill a cop because I "feel scared"
Your (justified) frustration with law enforcement does not mean the moral rules change, nor does it mean our adversarial system of law is wrong.