Spoilers ahead for Episode 2 of Outlander Season 5. Stephen Bonnet was, unfortunately, revealed to be still alive in Outlander's Season 5 premiere. And now, it seems he might weasel his way back into Brianna's life once again.
After Lord Grey mentioned rumblings of Bonnet lurking around the Provence in the premiere, we see him in the flesh during the Feb. 23 episode. And he seems to have one thing on his mind: seeing his son. When a comrade remarks that it's "not like" Bonnet to restrain himself from killing a man after he gets into a duel, Bonnet replies — quite sinisterly — that he's trying "to set a better example" because he's a "father now."
This implies that he's not yet done wreaking havoc on the Fraser family, which aligns with what happens to Bonnet in the Outlander books. Though the show departed from the Diana Gabaldon series it's based on in that it left Bonnet's fate up in the air at the end of Season 4 (in the books, Bonnet actually helps Brianna lead others to safety before the prison explosion), he does continue to be a menacing presence in the Frasers' lives. (Warning: book spoilers ahead.)
In the fifth Outlander book, Drums of Autumn, the Frasers encounter Bonnet again when he attempts to steal gold from Aunt Jocasta at River Run. He's able to escape before being captured, but Jamie is set on finding and killing Bonnet — something that enrages Brianna. Roger joins Jamie in this pursuit, but when they try to track Bonnet down at Wylie's Landing, he instead finds Claire, Marsali, Brianna, and Jemmy in Wilmington. He attacks Claire while trying to abduct Jemmy (who he believes to be his son). Claire encourages Marsali to shoot Bonnet, but she misfires. Brianna then has her own chance at deadly revenge: she tells Bonnet Jemmy will never be his son, then successfully shoots him. Bonnet flees the scene, leaving it unclear if he survived (so the books do have a Bonnet cliffhanger, it just comes a little later on).
Bonnet re-emerges once again in the sixth book, A Breath of Snow and Ashes, and that time, his wrath is even more menacing. Considering Outlander has already been renewed for Season 6, that means we may have yet to see the worst of this merciless villain.