I'm wondering if Nick's occasional turn to "the dark side"(like in ABRTI) was in any way prefaced by what his psyche brought up during his hallucination; that is to say, I was thinking that it was interesting to see a cheerful coroner or two, and his father calmly looking at the remains of his son, and smiling even, waiting to hear what caused his death.
The whole thing led me in one direction, albeit in a two-fold sort of way. First since Nick's imagining himself as dead, it seems natural that the familiar faces of the coroners would appear. But their light-hearted and even jovial treatment of Nick's body seems to indicate that his body just isn't something they take that seriously. And so I sort of extrapolated that maybe he doesn't always feel like everyone at work takes him seriously- the "bastard stepchild" we always hear him referred to, thanks to Q.T.
Secondly, his father is also quite jovial and seemingly fine with his son's death, and even comments that he'll look good for the funeral and his mother will appreciate that, as if that's all that's important. This combined with the doctor handing him the heart and saying "Your son had a good heart" and them both nodding and smiling seems to play on that worthiness again, but this time not just by the people he works with, but also from his father now. So maybe Nick feels like he's only thought of in terms of his empathy and being a "good guy" but not much else. As though this is/was his only real value in life. Maybe he felt like he not only possibly disappointed Grissom, his pseudo-father, but his real father as well, for whatever reason. Maybe because Nick took the criminalist job, and not the "doling out of justice" type job- i.e. cop/lawyer, Nick's father thought he was "soft". And being a judge from Texas, he was probably a reasonably hard man. Maybe Nick felt like he let him down because of that.
Obviously it's hard to guess more than that without knowing a little more about Nick's upbringing and his actual relationship with his parents, but that's sort of what I took away from that scene, on top of just the general Tarentino freakiness.