...and his loyalty and its explaination is explained in the book - giving weight to why Neri was Michael's Luca -

in both cases a man is backed into a wall and given an out by their respective Don's in exchange for loyalty and limitless violence if called on. Neither man drifted towards the Corleones and both started as the feared right hand - a wild card...

granted Neri went on to use his brain a bit more than his deadly skills, but the business changed quite a bit over the years