Complex sentence: Doug catches up with Ruby and the baby, and Mars follows them into the hills, but as he tries to kill Doug, Ruby interferes by putting a rattlesnake on Mars' neck, enabling Doug to overpower him.
Split into simpler sentences: Doug catches up with Ruby and the baby, but Mars follows them into the hills. As he tries to kill Doug, Ruby interferes by putting a rattlesnake on Mars' neck, enabling Doug to overpower him.