Some people prefer to trust solid facts first, rather than their own personal instincts. Of course, these facts haven't exactly been presented yet, but nonetheless, they prefer to put their faith into the system: That Sony wouldn't try something so stupid; that the Estate is trustworthy because Michael chose those executors himself; and that the odds would be entirely against them, if they stepped out of bounds.
People don't see eye to eye all the time anyway, so there's bound to be some who will draw the exact opposite conclusions from the same method (that is, just by listening). This can't be the case in all situations though, because if one were to hear, say, Justin Bieber singing a Michael Jackson song, we should all be able to reasonably tell that that is not Michael's voice. So, I think it has to do with how does a fan recognise Michael's voice? What is it that a fan listens for? Personally, the most important thing for me is Michael's style of singing. There's just something to it, that I can't put into words. That straight away when I hear even a soft ballad like KYHU, within three lines into the song, I can feel that something isn't right. But others might look for other things, like his pronunciation, his vibrato, his timbre, his tone, pitch, comparison to previous works, anything. This might be one of the reasons why fans can't agree with each other, other than the first reason stated.
I think the reason why they might get all agressive and hateful is because they don't exactly have the solid facts in hand. They can only speak of it in hearsay. If they had it (the forensic musicologist reports, that is), they would have probably tried to educate others of why it's Michael, since the solid facts say so. But since they don't, they get tired of being pressured into presenting something they don't have. They get irritated everytime the same thing is brought up, because they have no more solid evidence than the other side have. And so, it turns into hate, personal attacks and petty fights, because there's nothing more to offer.
Of course, I'm just basing this on my experience at other boards, in which when people don't have evidence to back up their claims, they become agressive, they start insulting others' intelligence or they start nitpicking at others' spelling and grammar.