CHACHAM…. I am not sure if this is right, but
A knows who did it, but not killer
B full truth- not there at all, so not murderer
C 1/2 truth, not killer but A was there…
D Liar, killer and A is lying (if A is lying means a half truth, since a half truth is equal to a lie…)
Am I close? this is a great riddle! 🙂