Unfortunately, a knife through the lobster's head is mostly a symbolic gesture for our own benefit. The lobster itself lacks a complex nervous system capable of transmitting feelings of pain. Thus far, there have been no studies that have been able to indicate that a crustacean's response to sensory input, that we would perceive as the animal being in pain, actually correspond to our idea of pain. The closest that a study has come to being able to show that crustaceans feel pain is being able to demonstrate that they react to sensory input, and can potentially learn to avoid such inputs. Responding to sensory input in a reactionary way that is beneficial to the survival of the creature ultimately the species is not something that relies on our definition of pain.

What constitutes death for a lobster? Does putting a knife through it's head cause it to lose whatever consciousness it had previously? I've worked with many lobsters in my time, and I've seen them struggling even after being dressed (and even partially cooked). I don't think that we have a clear enough understanding of a crustacean's consciousness to be able to definitively say that putting a knife through it's head is enough to kill it, in a manner that is humane enough that the majority would not feel uneasy.

I respect that your opinion, and will be happy to leave it at that.