This may be an exercise in futility, but let's build it and see if anybody comes.

We know that Opal, Krystal and Aidan were together at the gate of the Chandler estate when the shot sounded.

We are told that Adam and Annie and Emma were together in the attic, but we have only Annie's word, because Adam doesn't really remember. JR claims to have encountered Annie before the shot, but to be passed out at the time of the murder.

Zach, Kendall, Ryan, Liza were all on the estate and claim not to have done it, but had access to gun or had guns and have no alibi. Tad came out onto the terrace immediately after the shot was fired and picked up a gun there but, thanks to his pal Jesse blasting him in the head, he has no memory of whether he shot Adam or saw someone else shoot Adam.

David, Scott, JR, Little Adam, Colby were also on the estate, but no one seems to be questioning them too closely about the murder yet.

Jake and Angie were at the hospital, and Jessie was traveling from the hospital to the Chandler estate. We don't know the location of Nurse Hot-to-Trot, who was supposed to give David his alibi. We don't know where Marissa and Amanda were, even though they had no particular reason to kill Adam.

We know that Krystal and JR both tried to shoot Adam, but couldn't. Kendall claims to have tripped over someone or something, that may have been the murderer hiding out in the bushes on the estate. Liza went to the dead body, recognized it as Stuart wearing Adam's jacket, and retrieved her letters from the jacket pocket before hiding in the tunnels. Adam is starting to get back his memories of that night and may yet have more clues as to what happened.

Popular theories advanced on other boards:
1) Nurse Hot-to-Trot did it, after Adam fired her
2) Little Adam (or one of the kids) did it -- there were guns everywhere and he may not have realized it was a real one
3) Someone the viewer does not know was on the estate did it (maybe someone returning from the past)
4) David Hayward did it
5) Tad saw who did it, but is repressing the memory because it is someone close to him
6) Adam did it, in a confused drug haze

So, what's your theory?