During his time of Obito being with Madara, the White Zetsus told him that Rin and "Stupid Kakashi" were in trouble. Obito hurried as fast as he could to get to where Kakashi and Rin where. Once he got there, he saw that Rin was already dead and that Kakashi was the one who killed her. At the same time, Kakashi and Obito awakened the Mangekyou Sharingan which made Kakashi faint and Obito just sad/mad. Obito got so mad he went on a rampage. He killed all the other people that were around Kakashi and Rin. Once he finished, he went to Rin and hugged her. After that, he looked back to see Kakashi who was on the floor.
At that time, Obito had a huge chance of killing Kakashi, but for some reason, he did not want to kill the person who took the love of his life away from him.
Why did Obito spare Kakashi? Why didn't he kill him instead? Did he just feel bad for killing someone who was defenseless, or because he was going to feel bad for killing his old friend? Is there another reason for this, or is there just none?