Boring.

Also, If Shermie isn't using her melons for offence and defence here again I'll be disappointed.

Question: are these guys actually evil while pretending to be good or do they just get possessed in their Orochi forms?

Even in games where they weren't alive anymore, Chris and Yashiro definitely have some sadistic or hateful vibe in how they speak, even though they behave like you would expect some average popular singers - and they're no different from the period where they were alive. (I don't remember much about Shermie, she's mostly just playful)
I'd say they were already at least bad people (the kind who would say "I wouldn't mind if a plague wiped out most of humanity" with a big smile) and they simply have a job in the entertainment industry with good social skills. Psychopathic tendencies in the pathological sense, basically. They're not genuinely good people who like to hang out with others but suddenly changed when they learned who they were. Most of their regular speech sound like average banter if you ignore the Orochi forms, but it just seems like the Orochi versions simply crank up the same rhetoric to the max.

Oh, regular Shermie has a battle quote against Chris and Yashiro saying "this is where it all begins... and ends", which actually hints that they do know between themselves that things will end in death, even in their regular form.