First off, we know that the person who kill him wanted money. Therefore it could have been Josiah, who has recently graduated, and goes to Japan for work.
Secondly, we know that the person is tall. To throw us off, it was probably someone on stilts, so it was probably Silver who is known for his ingenious insanity.
Thirdly, we know the gun was made of silver. Therefore, we know that the killer is someone who knows classy metal, and it could easily be Metal.
Pi-thly, we know that the person was uncaring towards Winter's pain. As we know food tends not to care terribly much, be can assume Cheese could be the apathetic killer.
I-thly, we know that the person was working in a mob like organization. Phonox has already been proven to be strongly affiliated with the mob, so it is likely him.
Negatively, we know that the person likes to kill. This implies that they do it often, which is a trait that poison has, so it could have been PoisonWing.
Frankly, we know that the person was male. As far as we know, BFC is male, so it could have been him.
Lastly, we know that the person was working for someone called "the Boss." So it was probably someone who is easily misled into crime, like Poik.
With all these facts given, I must say that it was probably this smiley ->
How can you look into those eyes and not declare them guilty?
