Memoriam Speculation
Having watched the promo for Memoriam about a squillion times, in the spirt of true procrastination, I am going to give my theory on what is going to happen. Mainly so afterwards, people can point and laugh and say "You thought what??? Ha ha, you suck!"
I don't think William Reid is a murdering paedophile. The promo seems to be setting this up which is another reason not to believe it. As TPTB have pointed out before, the scenes in promos are not necessarily in the right order. And with this lot, some of them never actually make it to the screen.
I don't think the killer was Diana or Spencer either.
At the moment, some of my money is on Reid's Uncle Daniel whose funeral he attended as a small child. (And I don't think anybody murdered Uncle Daniel either. This is Criminal Minds, not a Virginia Andrews novel.)
So one theory is that Uncle Daniel killed Riley and Mini!Reid found the body or witnessed it. Uncle Daniel then killed himself. Mr and Mrs Reid knew but didn't tell the police because the perpetrator was dead and they didn't want to traumatise Spencer any more by him having to give a statement.
Or if it wasn't Uncle Daniel, who could be a dead red herring, Mini!Reid was at least a witness to something significant. And the anger at his father, "tell me the truth" is about his parents pretending that Riley was imaginary in order to protect Spencer from what he had experienced.
And, of course, I reserve the right to be COMPLETELY AND UTTERLY WRONG. *g*