[Discussion] Why did Kakashi had to open his left "sharingan" eye to kill Rin.

[Kurama999]

When Obito rush toward the battleground.
He saw Kakashi killed Rin.
Then both Obito and Kakashi eyes activated the sharingan.

Then why must Kakashi need to open (remove) his left sharingan eye, in order to kill Rin.
What did Kakashi saw or sense something about Rin.

for Discussion.

 

[Sussanoo]

He was already fighting.

 

[Kratos of cruxis]

In order to see your opponent while using chidori you need sharingan the manga already explained this