I'd love to know what his so-called 'aggressive' behavior was. Different people often have different definitions. Maybe he did have issues but that is a huge change from him being 'a great dog!'.
This is also another example of why it is so important to get dogs out of these shelters as soon as we can. Even if they don't seem to be in immediate danger of being put to sleep. Many dogs don't hold up well in the shelter sort of situation and can get 'kennel crazy' or show behavior that they would NEVER show in a home situation. Imagine how crazy you or I would get after a few days of being locked in a small area, nothing to do, and surronded by people yelling and carrying on. My temper would certainly be at its end.
Poor boy, hopefully now he can rest in peace.