My 'argument', as you call it, was a straightforward extension of your comment about computers only being capable of "acting like" they feel pain.

How would you show that a silicon life-form lacks subjective experience?

You said :

What is "pleasure" and "pain"? Ultimately, they're just electrical signals triggering the release of certain chemicals in the brain, with subsequent alterations to how the brain processes other stimuli.

My argument then is that if all pain is to you is a set of electrical signals than why should one avoid causing it. We don't usually have moral qualms about generating electrical signals.

I can't show that a silicon life-form lacks subjective experience anymore than I can prove that a human possesses it. What I know is that I have subjective experiences and that there are certain things, like pain and death that I really like to avoid. Since other human beings are very similar to me and also exhibit behaviors in the presence of painful stimuli similar to my own, I assume that they also possess such experiences. I refrain from causing them unpleasant experiences because doing so would tend to create a world in which such experiences are more likely in general and thus make it more likely that I would in turn undergo such experiences.

Consider this thought experiment.

Suppose that in the relatively near future we create a machine of equal or greater intelligence than humans. Suppose also, for sake of argument, that, for whatever reason, these machines do not have subjective experiences, but behave as though they do to facilitate human interaction. Would such a machine consider killing a human to be wrong ? Why should it ? The human claims that it fears death and wishes to avoid it. The machine also claims to fear death but in reality feels no such fear. If the machine has been told by humans that there is no difference between it and them because both can be reduced to sets of electrical signals then it could only assume that a human, despite it's behavior, experiences no actual fear of death and therefore that killing a human is not wrong. So it seems to me that there is a real danger in making the assumption that any sufficiently intelligent machine is sentient.

Is there a meaningful difference between a machine that claims it fears death but in reality doesn't (for some definition of "fears"), and something that actually 'fears' death?

How can you tell whether something 'fears' death or not?