"-Spike had actually killed a person. Strangely, Sam found that he didn't care much about the death of Marco. Maybe because he just couldn't just bring himself to believe Spike was a murderer. He'd have to see it with his own two eyes, though he wasn't sure he really wanted to."
Spike and Sam are still children, half the time still not quite sure how the world works yet. With Spike being sexually, mentally, physically, and emotionally abused since the age of 4 or 6 years old (by the only family he has - adding more confusion and trauma quickly over time), he will naturally bottle his problems up as that was his only way to save himself from worsened abuse from his uncle Charles. Killing Marco scared Spike shitless, but do you really think that after having someone rape you since you were little, living with that predator and facing the constant abuse everyday and keeping it all inside, that he is just going to just willingly announce to anybody that he killed somebody? His uncle would KILL him, or worse..
And as for Sam, he is still very much a young,happy,confused about life child. Suddenly he realises that he is actually in love with Spike, and doesn't understand his emotions and feelings. Worried about rejection from both Spike, his best friend, and his very loving father. And all the sudden he finds out from his best friend that they killed someone, and now that he knows about your feelings, Spike never wants to see him again, ever. Sam lived in a very healthy and happy environment for a child (as best as his father could do), so suddenly all these things are happening and as soon as he has them figured all out he gets rejected fairly hurtfully (but on Spike's side of the story, his reaction makes sense, seeing as his uncle threatened him if he ever fell for anybody but him. And of course having these things done to Spike against his own will almost everyday will make him have a distorted view on homosexuality), it all crumbles apart in front of Sam in seconds. All these things probably weigh on Sam a lot, and as what seems to be Sam developing depression, Sam probably won't feel the need to talk about something that he doesn't want to even think about let alone tell someone.
So since they haven't told anybody yet (as of this chapter), it makes sense nobody has addressed it yet.