Depending on the nature of the virus, I actually think that rage accelerating it´s take-over makes plenty of sense.
a) Think of how much adrenaline was pumping through his system when he died.
b) He was exposed to the virus for a longer period of time than previous character deaths that didn´t bite the dust with a gunshot to the head.
c) His mind was already wired to be a ruthless killing machine. When he died, he wanted to murder his best friend. It would only make sense that him waking up with the desire to eat his Rick until he was dead, would not take all that much re-wiring in the brain.
d) I am going to assume that the virus has similar properties to rabies.. just my theory. Lets see, excluding things that are obviously only brought on by a bite, lets examine which of these symptoms Shane seemed to already be showing.
The actual time between infection and when you get sick (called the "incubation period"
wink with Rabies ranges from 10 days - 7 years. The average incubation period is 3 - 7 weeks.
Symptoms may include:
Anxiety, stress, and tension
Drooling
(debatable)
Convulsions
Exaggerated sensation at the bite site
Excitability
Loss of feeling in an area of the body
Loss of muscle function
Low-grade fever (102 degrees F or lower)
Muscle spasms
Numbness and tingling
Pain at the site of the bite
Restlessness
Swallowing difficulty (drinking causes spasms of the voicebox)
I cannot say if Shane was trudging around with a fever or not, but his behavior was, slowly, becoming more and more like that of a rabid dog. Just hadn´t gotten to the convulsing, spasmatic stage. If we consider his feisty behavior to have been an accelerated manifestation of the virus to begin with, his fast-paced change into zombie-dom makes a little more sense.