That is horrible, so sorry, - I agree with the others, that one of the other fish did it. It may also be one of those rare coincidences:
Male bettas are very aggressive towards the female when spawning, & can often lead to her death. This could With be unrelated to the guppy/molly.
Livebearers can die because of the stress when pregnant/giving birth, & the Molly was probably just eaten by one or all of the other fish after she died, could have been from being harassed by the others & mollies can be aggressive. Pregnant livebearers are very vulnerable & a target for the other fish.
I hope it doesn't continue to happen!
I agree with Lemi & I would move any further livebearers when they are due to give birth.