What would be the Mane 6's reaction to TCB? Both misanthropic and alternative TCB

Most likely revulsion.

Very much sickened and they'd be very much wondering if they themselves would do something so evil.

If they were reading it as stories on the Internet?
Simple. Like everyone else does: cringe.

They'd probably wonder what the heck they just read, and they'd ask why.

Read “The Other Side of The Spectrum.” It kinda does tell a similar story. Although I advice you to read the reboot version “Spectrum.”