Robert Greene argues empathy (see Empathy as a Trainable Muscle) is not unbounded — it is possible, and dangerous, to go too far with it. The failure mode centers on toxic personalities: aggressive, narcissistic, grandiose people who don't announce themselves as such. On first meeting they are typically clever, charming, and seductive, and often display real pain — pain that specifically draws out other people's empathy.
Toxic people then prey on the openness that empathy creates. The harm doesn't come from their traits alone; it comes from the empathizer's own failure to set a boundary once that openness has been extended — you become too vulnerable and can no longer distance yourself. Greene calls this outcome "infection," a term he says he introduced in The 48 Laws of Power: the negative side of empathy where you become entangled with an affecting person and can't disentangle yourself.
The governing rule: you can understand another person's problems and try to help, but you cannot let their problems become your own problems. Greene applies a graduated trust criterion to decide how far to extend empathy — with colleagues or friends, where trust is already established, empathizing fully with their pain is a positive. With a stranger newly entering your life, more wariness is warranted: assess whether they can be trusted and have good intentions before empathizing further ("can I trust them with involving myself emotionally with these people"). He points to his book The Laws of Human Nature for methods to identify toxic people before getting involved with them.