First, welcome to the board.
Second, you have seen something horrible and are understandably very upset.
But, your parents are not to blame, and have no conception of the anger you hold inside. They, like many of us, were drawn in by thoughts of a loving God making this earth a cleansed paradise. They want wickedness such as you observed to end, and for all people to be loving and peaceful.
The other side points that the WTS makes, like the rape policy, which they went back and forth on (the woman must scream, no, she doesn't have to, yes, she does, no, she doesn't) aren't well known to the average person sitting in the Kingdom Hall thinking about petting a lion in paradise. Or, maybe they were concerned, but I think the latest belief on this policy is that the woman does NOT have to scream. Maybe they were relieved by this change, and felt that "the light got brighter."
The WTS practices shunning. It is a cruel and unloving practice, akin almost to killing the relative. It is treating them as though they were dead. Please do not do this to your parents.
Patience is something that we are not born with. One can look at a child and see this. It must be developed. But if all people excercised it, wouldn't this world be a far better place?
As a parent who believed with all my heart for over thirty years that the WTS was correct on every word that they pronounced, and raised my daughter in the religion, only to have her treated in a humiliating, cruel and heartless manner by a Judicial Committee, and called a liar and other things, I can tell you that your parents are truly convinced that they are doing their utmost to serve God in a way that they feel inside is the only way to serve him, but have not seen the other side of the WTS.
You cannot change that overnight or with one heated ultimatum. Please slow down. Anger is a horrible thing to live with, and an angry person does not usually persuade anyone with angry words. Calm and reason are called for. Sometimes, until one experiences something themselves, such as you did, they have no conception.
I am sure your parents don't. Please don't push them out of your life. I wish you the best.