But then, if your time travel was an accident, then you might encounter an interesting dilemma.
An interesting one was posed in a book I once read (can't remember which) when some people ended up in the late cretaceous, on an Earth populated by dinosaurs, and two small alien colonies, one peaceful, the other warlike. They persuaded the peaceful ones to help them steal an item from the warlike ones that would help them return to the present. They one, but they left the planet and redirected an asteroid to impact on the planet, directly on top of the peaceful race. The humans could give their stolen device to them to destroy it, but they realised that this was the very asteroid that wiped out the dinosaurs and ultimately allowed humans to evolve.
A silly story, but the point is in there. Somewhere.
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