Why did Kakashi kill rin?

Maybe cause he want to take the mankeyou sharingan or because was a zetsu clone but i think that is the 1st one.... OR

she was a spy for the hidden mist

in manga when kakashi killed rin the anbu guy said he's on to us(that means kakashi must have found out ) and the other guy said they worked so much to get it (he said much not hard so it means that the worked so much to train rin) the reason why kakashi was crying was because he broke his promise to obito.