Bode getting a lightsaber by taking it from Dagan Gera's corpse serves as a contrast to how Cal got his lightsaber. Both Cal and Bode were survivors of Order 66 and it's likely that like Cal, Bode lost his lightsaber when the Order went down or if not, he got rid of or hid his lightsaber to fake his death like most other survivors to better hide from the Empire. Both Cal and Bode got a new lightsaber from a recently murdered Jedi Masters (Jaro Tapal in Cal's case, Dagan Gera in Bode's case) and both lightsabers were even originally double-bladed and were used as such before they were killed. The difference is that Cal's lightsaber was given to him by Jaro Tapal before he died, while Bode stole Dagan's lightsaber from his dead body after he and Cal killed him.
Bode's turn to The Dark Side is a classic example of how most Jedi fall to the Dark Side.
Fear leads to anger: he feared losing his daughter so much that he got lost in a paranoid rage trying to protect her.
Anger leads to hate: he gets so angry trying to protect his daughter that he tries to kill Cal and Merrin with murderous hate.
Hate leads to suffering: In trying to protect his daughter, he's physically hurt and almost killed her, he's betrayed and/or killed so many people, some of which genuinely considered him a friend, and he dies alone as his daughter's become disillusioned with him and his former friends who ended up killing him have nothing but anger and disappointment in his betrayal and the man he's become.
Between this game and Obi-Wan Kenobi happening in the same year, Vader had a pretty rough time. If Obi-Wan defeating him decisively proved that Vader wasn't invincible, then Cere's near-miss probably proved to Vader that even a Jedi he considers beneath him can be a threat if given the time to train. This would explain why he dogged Ezra and Kanan so badly in Season 2 of Star Wars Rebels (and why he knew how to counter their attempt to drop walkers on him, because Cere already did that to him), and why he'd eventually develop fully into a No-Nonsense Nemesis the closer to the Original Trilogy we get; if Obi-Wan was a lesson in humility, Cere was a lesson in the consequences of recklessness and underestimating even someone you consider beneath you, a lesson he was no longer arrogant enough to ignore after limping away clinging to life.
