right then im here to explain my theory lol, kakashi had to kill rin.. (his best friend since obito was out the picture) so has now obtainted MS and since obito saw it im guessing he got it at the same time as u can see in the manga

right then obito went mental because the one thing he wanted in life was to save rin, like you havent really heard much about what happend to obito or anything but every time he wants to just protect rin, but he couldnt do that so he kills all of the bloody mist ninja with wood style he has got from the half of him he got from the tree at madara's cave.
and because of all of his hatered right now he thinks he is in hell and this next bit is just a guess but i saw black trees im guessing his hatred has mixed with the wood style to make the black zetsu ? im guessing ? i dont really know tbh

but i think thats pretty logical ..