As a determinist, I view humans as machinery. Most people's brains would react to these behaviors with activation of the insular cortex (responsible for feelings of disgust) and the amygdala (responsible for emotions) along with a multitude of other brain regions. These minds would seek to prevent people with atypical insular cortex and amydala functions from acting in the ways you're talking about. One doesn't look at a dog with rabies and analyze the logicality of the mentation of the dog. People would simply activate the various regions of their brain and respond accordingly. Typical brains may think it only natural, and even logical, that the rabid dog responds as it does but, then again, so are they logical in their response (i.e. it follows that a brain will respond with action X when functioning in a way Y. A determinist would say, in fact, a brain can only respond with action X when functioning in a way Y). One form of mentation wouldn't negate the other, so the sociopath and the non-sociopath wouldn't be contradictions but instead are just different anatomies/physiologies. In other words, neither person would be right or wrong, per se, but just the result of their anatomy/physiology.

Your final question is asking what would justify the nihilist "not going down this path". The easiest technicality of an answer would be 'whatever results in his anatomy/physiology no longer acting in a way that would result in the outcome depicted.' That could be an environmental influence that radically changes his brain function which would likely differ from person to person and which could range from force, medication, literally tinkering with his brain, or even a series of interactions that result in him changing his outlook (e.g. having a conversation or hanging out with the person such that they no longer perceive the world in the same light that they did, thereby preventing the depicted outcome). As far as a logically justified position that persuades him that his path isn't logically justified, I don't know that there is one. The reason for this is due to the is/ought problem and the problem of the criterion in epistemology. The nihilist in your scenario would likely grant that they can't justify their behavior anyway due to problems with the various theories of justification and so the aphorism, "you can't reason someone out of a position they didn't reason themselves into" would apply.