Don Fanucci reacted in the ways you have listed for the following reasons:

1. He barely fights back
2. How did everybody know?
3. Why did Vito hide himself?
4. Why was Don Fanucci never guarded?

1. Fanucci is shot in the heart with the first shot and slowly dies, a bull of a man. (the average person would of been dead before they hit the floor)

2. Fanucci was a Don and boss of the neighbourhood. He was more famous than anyone else virtually. Infamous may be a better term. The only defense against all Fanucci's aggression was to bulletin each other about him. People are very sensitive to being controlled under threat of death.

3. Vito hid before the killing of Fanucci because Fanucci was his own Luca Brasi (Vito's enforcer before he was shot) that is he did his own enforcing to the extent that Vito was metaphysically intimidated by him. This was all kept a secret like all the hits as death is always the payback from someone for these kinds of aggressions.

4. Don Fanucci seemed to provide his own secuity. He seemed to be far more active, criminally (the night in the theater) etc... than most of the mob guys. He probably could of easily hired bodyguards if he felt he needed them. Vito scored by calling Fanucci's bluff and winning. Vito probably was suspected throughout the mob as the hit man, eventually, and this was a major incident in which Vito deepened his involvement in the criminal underground.