Keep in mind that each individual's motivations can vary, and only a trained professional who is familiar with the specific circumstances can provide a more accurate analysis. However, some possible reasons could include:
Seeking validation and control: Narcissists often have a strong need for validation and control over others. By involving you in stalking his new supply, he might be trying to maintain a sense of power and control over both of you, especially if he feels rejected by his new partner.
Jealousy and revenge: If his new supply left him, he might be feeling angry, hurt, and jealous. Involving you in stalking could be a way for him to vent his frustration and seek revenge against her by making her uncomfortable or causing her distress.
Comparison and self-esteem: Narcissists tend to derive their self-esteem from external sources, such as admiration and attention from others. By comparing himself to his new supply and involving you, he might be trying to reassure himself that he is superior or more desirable than her.
Emotional manipulation: Narcissists are skilled at emotional manipulation. By involving you in stalking, he could be attempting to make you feel responsible for his emotional well-being, guilty for his relationship issues, or to create a sense of closeness and loyalty between the two of you.
Continued control over you: If he was controlling during your relationship, involving you in stalking his new supply might be a way for him to continue exerting control and ensuring that you remain emotionally invested in his life.
Lack of empathy: Narcissists often struggle to empathize with others and understand the impact of their actions on others' emotions. Involving you in stalking might not register to him as inappropriate or hurtful behavior.
Remember that engaging in stalking behavior is illegal and morally wrong. If you find yourself in this situation, it is crucial to prioritize your safety and well-being. Cut off all contact with your ex-partner, and consider seeking support from friends, family, or a mental health professional who can help you navigate through any emotional turmoil that might arise from this situation.