He really pulls out all of these very ominous and purposeful bits that only he'll get and can find amusement in, such as claiming that "not another living soul" knows "his" trick or that, to him, the theft of an idea is "worse than murder"...

... And then he gets rattled and completely squirms at the ever so-slightest threat from Gabriel. At the mere mention of him having a Grandad as an inspiration for his own interest in magic, Neville is so clearly on edge.

He also immediately folds at Gabriel's Mountweazel note and burns his secret of nine years - turning off the security camera so that there's no proof of anything happening. This is what leaves him with no alibi at the one point he actually needs it.

(Although I do think he could have come up with literally almost anything else to say when questioned about why he removed his security footage other than "no reason". It might not have helped much but it would have made him at least a bit less suspicious).

I think it's clear that Gabriel's plan relied on Neville's unyielding arrogance, but I think it's his paranoia that ultimately ruined him.

Plus, his own lack of creativity - literally keeping the name "A Chair Raising Experience" - also didn't help AT ALL.

Neville's security set up is honestly a really good reflection of himself and ultimately what dooms him. He has the security cameras set up at every angle to protect "his" ideas within the safe that he himself stole, whilst actively bragging in his published book about how well kept a secret he has. He wrote down the secret years ago and yet keeps the original copy and sets up this entire system around a point of potential exposure that does not even need to exist. He's paranoid at every opportunity at the idea of being exposed and yet cannot help himself but to show off.

Arguably, Squires does the same. He was ultimately pushed into a corner by Jacob when he had created a crossword about the current circumstances that easily acts as his own murder confession. Why? "A bit of fun" and to "teach" the girl who he knew was planning to kill him. It's to show off. However, this display of arrogance was done through a belief that he was in control of the situation. He was "aware" of the plan to kill him prior and so felt he could act in complete security and without consequence.