Anything outside the "official narrative" will be pounced on as a "conspiracy theory" by the many authoritarians here..
Such as;-
the suggestion that Guede was an informer that the police had been using in an ill-thought out strategy to deal with the spiralling drug problem in Perugia (which for a University town was particularly damaging), and that they left him on the streets despite obvious signs that he was becoming dangerous.
If so, the authorities themselves would be culpable in Meredith's murder, and there would be a powerful incentive to camouflage it by framing a couple of other kids.
The rather parochial authorities might have believed that stitching up a couple of students wouldn't attract too much attention.
Guede would thus still beleive he is in a position to, shall we say, bargain his way out of jail in a few years.
I agree entirely that this whole "police informant" issue could blow the whole case wide open. Now, of course this MIGHT just be passing into the realms of conspiracy theory - but on the other hand, as of right now there does appear to be at least a small grain of truth to it.
And if RG WAS an active police informer, then the argument that you've made flows very logically. Although I'd take slight issue with the part where you suggest that "stitching up a couple of students wouldn't attract too much attention". After all, one of these two students was a foreign national, so they'd have known that this would give the case an ongoing international dimension (the fact that another foreign student was the victim was sadly beyond dispute, thus ensuring some foreign interest, but minimising the chances of international criticism). And both AK and RS were from the educated middle classes, thus raising the spectre of ongoing protest and representation from their families, friends and/or support groups (in contrast, a degenerate drug dealer with perhaps no solid family unit, and probably with few articulate or reliable friends, would pose no such problems).
Instead, I'd suggest that,under your "police informant" scenario, the police/prosecutor would merely have had a powerful incentive to obfuscate and/or minimise RG's potential role. The fact that AK and RS also seemingly "offered themselves up on a platter" (as the police saw it) simply would have given the police the opportunity to focus attention on these two, as the lesser of two evils (note the specific idiomatic use of the word "evil" here).
In your scenario, I'd be pretty certain that - given a free choice - the police would have preferred to have collared one of the city's many drug dealers who were not also informants, whom they were gunning for in any case. However, once AK and RS placed themselves in the frame, and especially once forensic evidence started to turn up that could be consistent with AK/RS's involvement, this would clearly have been the police's best course to pursue - rather than continuing the search for a suitable drug-dealer suspect.
NOTE: this is ALL hypothetical, and based on the far-from-probable premise (as of right now) that RG was an active police informant at the time of the murder.
) that Guede would take certain others with him if he came clean, so these 'others' would have an incentive, to say the least, to exert whatever influence they can on Guede's behalf at his remaining appeal.