Arthur's Heartbreaking Diagnosis: Red Dead Redemption 2 [episode 15]

While going to a Sadie mission, the game ropes me into a cutscene of Arthur getting sick and falling off his horse. An unknown NPC helps Arthur to the clinic. Arthur learns he is dying of an illness that was (at the time the game took place) incurable.
Molly confessed to being the rat to spite Dutch, resulting in Ms. Grimshaw executing her.