He's telling Kakashi to not make that face, well half-face, and then to not be hasty about the blame. He probably didn;t want to get emotional then screw up.
He is basically mocking Kakashi here, as in "you killed her, and now you're making this face and trembling, heh, funny", or better "Remember what you did last summer, Kakashi? We both know, hehexd"
If I remember right, Obito talked about Rin's death in the previous page and said it wasn't kakashi's fault on the next page. I assume he meant "don't have a heart attack i don't blame you for this"
Op: I don't see the face....? Maybe it's because it's ALWAYS COVERED..
Op:Serious response: I I think he was trying to poke fun at him a little, ie the 'huhu' and the'dont make that face'.