There are several causes for us to hear the voice just inside the brain. Although, first of all, I do not want to discuss the deity, and moreover the supernatural including the devil. At least, God is unknowable to me beyond my understandings to discuss.
When the perpetrator mentions of hearing the voice inside the brain during the trial, there are four possibilities, hallucination, drug, lie or electromagnetic. In this case, it was less likely a result of telling a lie as there was no other explanation but the voice driving him to the mass murder. He clearly mentioned an existence of the voice at the time of the arrest, as testified by the police officer, and has never changed his side of the argument that he followed the voice to commit a violent act.
The drug was also not a cause of this incident as the evidence was presented during the trial that he was clean to the drug test.
The leftover answer is the hallucination or electromagnetic. It is actually quite hard to tell whether the voice is heard due to the hallucination or the real culprit sent voices through the electromagnetic wave directly to the brain. In some cases, the perpetrators have noticed that the voices were sent from the outside, though it was quite rare at least until now, as we had not realized yet that the electromagnetic technology was deployed to manipulate the subjects. Moreover, it is still not a common understanding that the voice can be sent directly into the brain through the electromagnetic wave.
I concluded that the voice was sent from the outside in this Guam murder case rather than the hallucination. He caused these murders within a few months after starting to hear the voice, which is one of the strong pieces of evidence. If it had been a hallucination, his mental health would not have been aggravated within just a few month even to kill and injure many people. This quick escalation should be quite rare for the case driven by the hallucination.
On the other hand, it is quite common that the operative can brainwash the subject to commit a felony including the murder within a few months, while it is quite hard to brainwash the subject within several days. They can access the brain of their target directly through the electromagnetic wave, though it needs some time to crack the ethical code for a further action. Even if we hear the voice inside the brain, we do not believe its command soon as it is basically outside of our thought process. However, they can change the ethical code through the voice sent by the electromagnetic wave, which should take at least several months.
This murder in Guam was less likely caused by the mental irregularity aggravated just in a few months as it was beyond the understanding, while it is quite probable that the perpetrator was manipulated through the electromagnetic wave to conduct a felony in several weeks of the brainwash. There was another reason for me to conclude this case as the electromagnetic manipulation that there were some connections to my case.