I do not believe that actually happened as regards "Computer Killer"; it just seems like it did. Perhaps the writer(s) realized there was not enough time in a 60-minute program to properly resolve it, so the last ten minutes or so is what had to be presented as the resolution.
In essence, a hot-shot computer programmer makes a deal with the father of a murder defendant - the hot-shot will get the young man acquitted for a large sum of money. The hot-shot brilliantly and cold-bloodedly, using a "classified" computer code-book, frames another man. How did hot-shot acquire the book? Nobody knows. There are many, MANY loose ends throughout the episode.
Alright, never mind. The PROBLEM is McGarrett, despite having a mountain of physical evidence, all of a sudden decides none of it matters. Instead, since the phone company, jewelry stores, the police, airlines, and banks all use computers to do business...the REAL killer is a computer expert! It is no different than saying "Hey wait...the killer drove a car...therefore he's a mechanic at a Ford dealership on Waikiki Drive!" If you think that sounds ridiculous, you now know how I felt at the end of the episode.