This silent Western is the earliest movie that I'm aware of that has a rattlesnake in it as well as a death from snakebite and a snake used as a weapon. It's unusual in that it was made in Las Vegas, which in 1910 was a small town with a population of only 800.

A flyer shown on IMDB explains the whole movie. Here's a summary of what it says:

Tony has a girlfrined named Inez. He becomes jealous of a man named Jose and yells at him.
Jose drops a rock off a cliff and knocks Tony unconscious. Jose climbs down to kill Tony with a knife but before he can, a rattlesnake bites him on the leg. Tony wakes up and Jose runs away. Tony watches Jose die in agony from the snakebite, and he picks up the rattlesnake.

Inez takes Jack to the place where Tony and Jose went. They find Jose dead and Tony laughing hysterically. Tony takes the rattlesnake home and makes it a pet. He goes crazy and becomes dangerous like the snake.

Inez tells Tony she won't marry him unless he kills the rattlesnake. She marries Jack and they have a baby girl.

Tony wants to get revenge on Jack for stealing his girl so he sneaks into their house and puts the rattlesnake in Jack's bed while he sleeps. Before the snake can bite him, Jack wakes up and chases after Tony. Tony returns to the bedroom and discovers that Inez has put the baby in the bed with the rattlesnake. Tony kills the snake, and doing that he suddenly stops being crazy. He hugs the baby and they all live happily ever after - except for the rattlesnake, of course.