Spoilers ahead for Riverdale Season 4, Episode 16. We already learned that Jughead wasn't really dead on Riverdale, but the March 11 episode also revealed why Donna tried to kill him — because, yes, she was the mastermind all along. Though the show spent the season painting Bret as the main culprit in this drawn out murder mystery, it was actually Donna pulling the strings. She devised an elaborate plan to take out Jughead without getting any blood on her own hands — and it would have worked, if Betty and Jug hadn't figured the whole thing out.
In the March 11 episode, Jughead and Betty locked the Stonewall students in a classroom and took them through Donna's scheme from beginning to end. Apparently, it's become tradition that new Baxter Brothers ghostwriters have to actually commit murder to understand what they're writing about. No wonder Mr. Dupont had to steal the Baxter Brothers idea from Jughead's grandfather; these people have no imagination.
Anyway, Moose was the initial target, but because Mr. Dupont felt Jughead was getting too close to the whole truth, he ordered Mr. Chipping's class to redirect their plans. Donna took the lead in organizing Jughead's murder, because she really wanted that Baxter Brothers contract.
However, her motive was ultimately revenge. It turns out that Donna's grandmother Jane Dallas Brown was in Mr. DuPont's secret society way back when, and that Mr. DuPont also stole something from her: the idea for Tracy True (Riverdale's version of Nancy Drew). In order to protect the secret of his intellectual theft, Dupont killed Donna's grandmother along with the rest of the society's members (minus Jughead's grandfather Forsythe, who ran and hid). As such, Donna spent her whole life working to get into Stonewall and Mr. Chipping's class so she could have a shot at getting the Baxter Brothers contract and rebranding it as Tracy True. Unfortunately, she became no better than Mr. DuPont in the process. Donna went along with the plan to kill Mr. Chipping, who was going to expose Mr. DuPont; she orchestrated Jughead's attempted murder; and it's implied that she also ordered Jonathan's death after he failed to confirm that Jughead was indeed dead that fateful night.
That's a lot of blood on Donna's hands, even if Betty is the only one who clocked her. In the end, Betty urged Donna to give up the book contract because "Tracy deserves better, as does the memory of your grandmother. And you certainly don't deserve to profit off the near death of my boyfriend." But she didn't really give Donna much of a choice: Betty has medical records that prove Donna was related to Jane Dallas Brown, which she can use as evidence to implicate Donna if she doesn't do what she says.
With that, the Jughead death mystery has finally been tied up, and the show is moving on to lighter fare: a variety show spearheaded by Kevin, which will like air in early April after Riverdale takes a short break. God knows we could use one after all of that drama.