Mind=Blown (⊙…⊙ )
Shook (⊙…⊙ )
Maybe the other ciel and his butler?
perhaps it is Lissy?
Or it could be dolly from circus. Because in the walls written, "candy" I remember dolly have the same eyes color as Ciel. The reason why Ciel asked Sebastian if he would lie or not to Ciel.
That wouldn't make any sense because Doll is dead.
**SPOILER ALERT**
First of all I think there were two killers not one...
One is with the gun. The second is with the knifes.
Look at where Agni is attacking:
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/127/pg-5/
But then he is attacked from the side with a knife:
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/127/pg-6/
And then the killer said that he'll take care of the rest.
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/127/pg-8/
I think if the killer had both knifes and a gun then he would had killed Agni with a gun and finished him off much quicker...
Secondly... I think the one with the gun is "Ciel" aka our Ciel's twin brother...
Why?? Because it was obviously someone the prince knew.
http://www.mangago.me/read-manga/kuroshitsuji/an/manga/kuroshitsuji/v18/c126/pg-19/
And then the writing in Ciel's room...
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/128/pg-13/
It is something really childish and somethings kids might use to play or something. Sebastian clearly has no idea what that means while Ciel seems traumatized. In other words, Ciel know of this statement since before he met Sebastian.
Later, Ciel is shocked to see his burned picture in Agni's hand but he doesn't seem to be upset that Agni saw the picture... He seems to have something else on his mind.
http://www.mangago.me/read-manga/kuroshitsuji/an/kuroshitsuji/128/pg-16/
Well, that's what I think anyway (〜￣△￣)〜