But where did the sex game gone wrong theory come from?
Unlike the US, where no motive is required, Italian prosecutors are expected to conjecture a motive. The first prosecutor conjectured one idea, the second one another. Conjecture about motive is not evidence, it's a vehicle to present the evidence.
If they'd had confirmation about the prank, perhaps prosecutors would have conjectured a motive around a Halloween Prank, similar to the 2007 April Fool's Day Prank.