On the topic of why didn't Officer Wilson "just try" a TASER:
TASERS launch two electrodes affixed with barbs. According to several law enforcement commentators that have given interviews on cable news channels these past weeks, both barbs must attach to the clothing of the subject in order for the current to flow. If one of the barbs fails to attach, nothing happens. Another disadvantage to using a TASER is that you only get one shot, and then you have to reload the gas cartridge that deploys the electrodes.
So let's suppose that Officer Wilson's version is factual, and let's put ourselves in his shoes for a moment: a 6'4" 292 lb. man has just punched you in the face and struggled with you to try to gain possession of your firearm. You are alone without anyone else to back you up or offer protection against this very real threat.
If that man runs away from you, but then turns around and begins advancing upon you again, are you going to have enough time to take out your TASER, discharge it, hope that both barbs attach to the subject's clothing, and hope that the electrical current disables him before he reaches you? If the TASER fails, are you going to have enough time to then draw your firearm in order to protect yourself before the subject reaches you?
Or, are you going to fire your gun (the same gun that the 6'4" 292 lb. man had only seconds before tried to wrestle from you), believing that if you don't shoot him, you may more than likely end up dead, shot & killed with your own firearm?
If Officer Wilson's version is factual, and if his version is corroborated by the alleged 11-12 witnesses (not to mention the unknown witness heard on Black Canseco's video), the proposal of "trying" a TASER first in the face of an imminent threat is not feasible, nor is it proper police protocol, IMO.
There is a time and a place to deploy a TASER. In my view, this was not one of them.