I found some youtube comments what people said about this chapter and this is what they wrote
Okay we have 4 possibilities.
1. Hidden Mist justsu Rin ran up on Kakashi and got Raiki'd.
2. Kakashi was made to see Rin as someone else then snapped out after he stabbed her.
3.Like I said, Sharingan sees through GENJUTSU so unless it makes the person a exact replica of someone else like Pains ability I find GENJUTSU unlikely.
4 Newest One I see. Zetsu transformed into Rin & Kakashi killed the WRONG one.
Another one is
Remember when Obito told kakashi, you let Rin die. That means he knows Kakashi didn't kill her so my guess would be that it's a tranformation jutsu to trick Rin into believing that its Kakashi when it's really a hidden mist ninja. That's why Tobi changed the hidden mist to the blood mist.