I previously stated that Guede doesn't fit the profile of a Satanist or a pod. However, I forgot to consider the possibility that he was overshadowed by the Spirits of Darkness when he committed the murder. It seems like he had no history of rape or murder (I haven't noticed anything about this, anyways), and Perugia is near a significant ley line. A mitigating factor is the time of year - November, when nature forces in the northern hemisphere are at an ebb. If he had just been caught in the act of a burglary, he probably could have just taken off with minimal consequences, considering the lenient treatment he had been getting from the cops. Why then did he opt to commit such a heinous act?
So, I don't know, and I'm not going to dig into it because Guede's not going to let us know what was going through his mind when he raped and murdered Kercher.