TL;DR: Rhaast was a scythe first.
The long version: I finally figured out a way to reconcile the two seemingly contradictory lore implications about Rhaast in Vladimir's updated bio and color story. Instead of being a Sunborn sealed into a scythe, Rhaast is a living scythe created specifically to murder people and possess weaker hosts if need be. His creator? Vlad's Darkin master.
The possession ability was meant to stop anyone aside from Vlad's master from wielding Rhaast, because if anyone else picked him up, he'd steal their body and kill everyone around him. Vlad's master was known for doing unorthodox things compared to the other Darkin. Why not add a living weapon to the list? Of course, with said Darkin master now dead, Rhaast's "kill people for the boss" purpose is gone. At least, the second half is. Rhaast was made for killing, there are not-dead people around him, it's not too hard for him to put two and two together and keep up with what he was made for.
Why build a living weapon nearly guaranteed to wreak bloody havoc upon your death? Well, this is Vladimir's master we're talking about. Anyone who didn't see their human pet's betrayal coming was clearly not good at planning ahead. Plus it echoes the common theme with the Darkin of one person's error screwing over everyone else. (Yes, I'm talking about Myisha's fiendishly clever 'leave our enemies alive and make them even angrier' plan.)
This brings together the "Vlad's banner in that painting had a scythe on it" detail and the "Vlad killed his master" detail. Rhaast didn't wield the scythe on that banner, he *was* the scythe.
-Aatrox is completely traumatized by about six seconds in sword form because his default body is one with arms and legs and lungs. Rhaast is seemingly unfazed and able to banter with Kayn because he's already in his default body. He still refers to the scythe as "confining" because he considers it way more fun to kill things himself than to be wielded by someone else.
-He says he was "forged to destroy" in his VO. I say what if that's literal?
-He's got a one-track mind with "MURDER" written on said track in large neon letters because that's what he was created to do.
-Rhaast's body looks the least human out of the Darkin we've got. That's because he's not trying to look like a Sunborn, he's trying to look like a literal living weapon's idea of scary and lethal.
-He's somewhere between unfazed and jubilant in his respawn quotes because he was built to last. Contrasting with Aatrox, who loathes his immortality, Rhaast loves it because it means he can always do what he was made for.
-Pantheon only talks to Kayn in his interactions with the two despite knowing who Rhaast is and speaking directly to Aatrox. This is because he knows enough about Rhaast to be sure that he literally is a weapon and not a person. (Now whether a fully sapient weapon can be said to possess personhood is a debate for another post.)
Is this the direction Riot is most likely to go? No. But it lets me staple together two seemingly contradictory lore bits and that's good enough for me.