In my view, they made two critical mistakes:
1) The bing/G+ mistake - letting other companies set an agenda that had them relying on execution outside their area of competence. They wanted to push phone contracts and TVs like walmart, but they lack the retail space and brand recognition to do so. This transition can be made (costco comes to mind), but Radioshack bet the farm on it and lost.
2) Maliciously pricing hardware components. I say "malice" very intentionally. Things like adapters, resistors, and pots cost literal pennies coming out of China. Hobbyists will happily pay $1 for a $0.10 item to avoid sourcing it from the factory or ordering from a catalog. $8 is a different story. Going to Radioshack went from "awesome, what will I build this week?" to "man, they're really fucking me on this".
Couldn't have happened to a more deserving bunch. Radioshack's death is long overdue.