Noun describing one who "acts on emotion primally"?

I’m looking for a noun to describe someone who “acts on emotion primally”, or something similar.

To clarify, I’m trying to make a point about how the fanaticism of tech-fetishes (iPhone fanboyism and the like) causes people to forgo rational, sometimes even calculated, intelligent responses to criticism. Instead, people lose themselves in irrational, primal competitiveness and negativity; responses near involuntariness and waste potential value.

If any further context is desired, please let me know.

I think the closest word I’ve come to is primal, but I’d really like a noun and I’m not sure primal is as precise as it could be anyway.

Bonus: If you have a good antonym for the word that describes what I’m trying to say (so it’s more along the lines of what I mentioned as rational, calculated, intelligent) I’d love to hear it.

You asked for a noun but then mentioned some adjectives, so I'm going to suggest visceral, which describes a 'gut response' as opposed to a reasoned, or cerebral, response.

Your question contains the answer: the person you describe is a fanatic:

a person with an extreme and uncritical enthusiasm or zeal, as in religion or politics

You could use evangelist although it implies that they are trying to "spread the word" rather than just blindly follow their beliefs. There must be another religious term that works, but I haven't thought of it yet.