just wading in: folks, let's keep this civil before the mods step in.
Derren: Watch the second Planet of the Apes movie. There are mutants with psychic powers. Heck, watch that Star Trek episode where Captain Pike gets zapped by those mind-control aliens. Psychic damage, right?
Now 4e is different in a lot of ways: Tieflings are a true-breeding race (rather than 2e planar mutts). Aasimar are now Deva. We don't have the same number of repetitive metallic dragons. The Abyss is in the Inner Planes (now the Primordial Chaos).
That said, is it too much of a stretch for one type of draconian to give off psychic energy that harms a slayer? Whether it's fear-based or just causes mind-pain of an unspecific sort, it's just a monster ability.
Also: the body's fear response does damage it, if you don't react properly or deal with the stress. If you live in constant fear you skip from your alarm-phase response to a more long-term "resistance" phase stress response: like being on a yellow alert in star trek tng. It's not at its peak, but if it goes on for too long everyone's body will start ot break down in some way (just about everyone has a "weak link" in their system, like a bad back or stomach, or heart disease).
So, quite literally, your fear (or fight-or-flight response from your sympathetic nervous system) can cause damage to you. It's not glowy pink energy like Xmen, but rather your body will kick into overdrive.
Maybe they give off fear-inducing hormones when they die. Or maybe "it's magic!"