I think this has more to do with Mara & maybe Petra, they knew the only way to get us involved in their war was to kill a vanguard member I.e. Cayde. Think about Mara and all she's conspired, shes been conspiring with the nine, she's tells the Emissary to have the Nine move the "asset" and then the only next scene we see with the Emissary is her talking to Drifter. Not only this, but his ship and Hunk are located near the Nine. The Drifter is just Mara & the Nine's pawn and, though he might have given the rifleman the Sorrow bullet, he's way too out of his mind (talking to himself, drinking vex fluid, paranoia etc.) to pull this all off himself. So you see why Cayde's death was both part of Mara's plan and the Nine's own plan. For the nine, it wouldn't be the first time they allowed evil to bring out the traveler and, for Mara, she only cares about the reef/dreaming city and will do whatever ensure that her people are safe. Neither of them care about the guardians/vanguard, we and the Drifter are just tools to them

The Mindbender had his Hive make one for the Rifelman. We legit fight a Weapon of Sorrow smith on the Tangled Shore that used to be a part of the Mindbender’s brood. My opinion.

Well he came hauling a ship. It’s just coincidence, maybe he did help plan the prison attack. Cayde dying was his arrogance, along with the ghost for revealing itself like that.

Uldren was corrupted by the darkness. He could have known how to craft such bullets