I would not go in with an alligator, even if I thought that he looked like he was sick. He might just be feeling hungry, and only needed something to eat.
That said, I really don't think that using a reptile to guard something is a good idea either. I have had geese and turkeys before, and either one of those will quite likely go after anyone that comes in your yard.
The problem with that is, you may not want your company getting spurred or pecked by a turkey, or snapped by the geese.
Another bird that is a great guard bird, but is not so apt to attack, is a Peacock. They will sound the alarm, and let you know that someone is out there, but they are not as apt to peck someone as a goose or turkey is, however, they will often stroll around the stranger, and give them a sharp look over, which will probably discourage anyone from wandering around much, before you can get to the door and let them in the house.
