Originally Posted by LonelyNinja
There was something out of place, but nice, about Kakashi talking to the Kyuubi like that. Like, "Oh hey, I know you've slaughtered thousands of people in the not-too-distant past, but I'm over it." Kakashi confirmed for most character development, fuck what everyone else says.
Actually it has never been shown or said that the Bijuu have slaughtered thousands. Just that they are great powerful beasts of pure chakra. Every conflict of the Bijuu that has been detailed are either them being captured, them breaking free from their "hosts" or them being manipulated.
Kurama especially the only contact he has had with the history of Konoha is being controlled by someone, he has never attacked them just because. Also it's not on record that so many people died. The most notable are Naruto's parents and Iruka's Parents. But in the later that could have been collateral damage. Kurama actually stabbed Minato and Kushina.
Kakashi is probably seeing it like; "OKAY if Naruto can be friends with him I can too. He is helping fight this great evil in front of us, after all"