I still think the theory that Lassie was trying to destroy Timmy fits the evidence best.
Lassie goes outside, Timmy follows.
Lassie leads Timmy to some horrid creature.
Timmy starts getting chased, Lassie excitedly watches until doom seems assured, then heads back to the parents.
Lassie then brags about how Timmy has finally been destroyed.
The parents somehow home in on Timmy with Lassie assured they will find his gruesome remains.
Timmy somehow still alive..Lassie is shocked.
Like I said: Lassie was a Laddie. For some reason, it was thought a male dog would be “easier to train,” and besides: with all that hair, who could tell the difference?
So trying to do away with Timmy could be explained by jealousy...yah, that’s the ticket: The dog was jealous. ;o]
*e-vil laughter*