It's easy to villainize the parents. Their ignorance and misguided intentions certainly did not help the situation, but whether they feel they lost a son or a daughter, they are not heartless monsters.
They are grieving in their own way and must deal with the conflicting thoughts and feelings - and I am sure guilt - regarding their loss. This is something they will need to live with. Please don't add to the pain by attacking them. The hate has to stop somewhere.
The problem is the rampant pseudo religious ignorance that fosters such intolerance. Is this truly the teachings of Jesus? Is there a place for intolerance and hate in religion? Is there a place for this intolerance in the office of therapists who are supposed to offer a safe non-judgmental space for their clients?
The sad reality is in their ignorance, they are not aware of their ignorance and truly believe they mean well.
Angry? Upset? You should be. But channel those feelings more productively. Support transgender rights and groups, and encourage acceptance of others. Direct intolerance toward politicians and organizations that foster hate and inequality.
Consider making a public statement on your social media of your support.
Remember also that while we tend to focus on the difficulties of the transgendered, it can also be difficult for family and friends. It's still relatively new in our society and many people are still unsure how to properly process and respond to such a revelation. Patience and education are needed.