Well so far, I have a list of 5 possible suspects in the Alexander killing
1.Mc's mom since she and Sarah are identical twins all she would have to do is dye her hair to look like Sarah's.
2.Aiko seeing that she was the one that was served up to the old fucker she would have the bigger motive to kill him and also, she did spike one of the wine glasses with a pill but who got the spiked glass it didn't show so perhaps Sarah got it since she was passed out on the couch while the murder was taking place.
3.Kate if Aiko told Kate about what happened and with Kate being protective of her seeing as the two are lovers, I wouldn't put it past Kate to seek revenge on behalf of her lover.
4.Jessica because she was the one who set up the deal with Alexander in the first place so she would have a lot to lose as well and then fucked it all up when the old fucker started getting greedy wanting more money and then she got all nervous and didn't have the stomach or the nerve to fix her own damn mess which her and Sarah argued about and also, it may be that Jessica might be jealous of Sarah's power and wants to take it from her and become the new dean of the college among other things.
5.Monique maybe?
I don't believe Monique had anything to do with the murder, but then again since Sarah's arrest she's been acting a little suspiciously erratic, but someone definitely knows something since the gun is now missing so that leaves 3 immediate suspects that know where Sarah keeps her gun stored.
Anywho those are just my personal thoughts.