This is great. A good build up of tension and it certainly kept my attention as I wondered what Alex is so afraid of, and why.
The prickle started between my shoulder blades and trickled down my limbs like iced water the moment I stepped outside the front door – I thought “the moment I stepped…” was continuing the simile so this pulled me out of the action a bit until I registered. Maybe start the sentence with “The moment….” I love the description of the prickle, though.
and the front of Auntie’s red brick house jumped out of the gloom. – lovely visual.
Not that I feared unknown demons in the dark; it was the monsters that looked like ordinary people that scared the shit out of me. – three lots of “that”
nobody or no-thing could reach me. – I think I see what you’re trying to do with the hyphen but it feels clumsy.
if it hadn’t been for the barbed wire that was uncoiling in my belly at the thought of my destination – great description
all because I had to walk into a room of strangers. A room where I wouldn’t know the exits and the quickest way out, where the barman would be a stranger and the regulars unknown. – this feels a bit wordy. Maybe delete “of strangers. A room”. This would get rid of the repetition of stranger too.
If they ever guessed how much I’d seen, how much I remembered, would they come for me? – I would have thought the answer to the question is a foregone conclusion given they are watching him. Maybe turn it into a bald statement “they would come for me”.
But I knew what I’d seen. – strong final sentence to hook us into reading on.