In the movie Hot fuzz detective buttermand threw his gun and it fired and hit the bad guy and i heard that if u drop a bullet it will fire and possibly injure or kill someone

It must have been a defective weapon. Modern hand guns are designed with a transfer bar so that the firing pin is blocked from touching the cartrige primer unless the trigger is held in the extreme rearmost position.

On a previous episode of mythbusters, they showed that without the gun barrel's support, when bullets are thrown into a fire, the casing will explode out before a bullet is fired, acting more like a fire cracker. Injury could result if the casing hit exposed skin when it exploded, but is highly unlikely that it would kill you. So, while dropping one could set it off, the bullet wouldn't be fired, the casing would just explode.

for most bullets to go off they got to have a pin to press the little circle on the back (the primer) but calibers such as the .22 are "rim fire" bullets that means that the rim of the brass is pinched to set off the primer. these rim fire bullets may go of if thrown at the ground but "not likely"