Wife send a wats up message that she will kill my son

It is always better to prefer an amicable settlement, wherein you should not succumb to the threat extended by your wife and proceed to persuade her so that she may rejoin and enjoy your company / matrimonial life. 

I understand that you're going through a difficult situation. Receiving a threatening message from your ex-partner is distressing, especially when it involves your child's safety. Considering your permanent child custody case is already in progress, it's essential to take immediate action to ensure your son's protection. Here are some steps you can consider: 1. Document the threat: Save the WhatsApp message as evidence. Take a screenshot or save the conversation. 2. Inform your lawyer: Share the threatening message with your lawyer immediately. They can guide you on how to address this in your ongoing custody case. 3. File a police complaint: Report the threat to the police and file a complaint. This will help create a formal record of the incident. 4. Seek a restraining order: Your lawyer can help you explore the possibility of obtaining a restraining order to protect you and your son from your ex-partner's threats. 5. Prioritize your son's safety: Ensure your son's safety by taking necessary precautions. You may want to consider changing your daily routines or increasing security measures. 6. Keep records: Continue documenting any further incidents or communication with your ex-partner. Remember, your son's safety is paramount. Please consult with your lawyer and the authorities to ensure you're taking the best possible steps to protect him.

She may send such threatening messages, but practically she will not do it out of fear.

if she has sent this message, what prevented you from showing the same to her parents and requested them to intervene and convince her.

you can express that you are always ready and willing to take them back to your house if she agrees for that otherwise you will proceed as per court order.

In the meantime you don't withdraw the case, let the court pass an order which will be a strong weapon to defend your insterests.

Being apprehensive of damaged matrimonial life she has sent you the message, which is just a threat, ignore it for the tiem beibng and proceed to resolve problem/ dispute / issue(s), if any.