Kakashi is under genjustu or rin was a spy the only two plausible answer's.if not then I don't know what could be the pausable reason for Kakashi killing Rin
post in the official discussion thread. If Rin was a spy I don't think Kakashi would mourn her death the way he does because she wouldn't be a comrade. Dont think its genjutsu either because that would be a cop out. Obviously Kakashi was forced somehow, but don't know how.
How about this: They were surrounded by a bunch of Mist nin and in the heat of the battle....Kakashi caught Rin by accident :O Seriously guys, does there have to be a purposeful reason for everything. xD