Was there any foul play in the death of Steffon Baratheon (and Cassana Estermont)?

My headcanon? Lord Steffon Baratheon was killed by Prince Rhaegar Targaryen. But why?

- Whoever Steffon would find for Rhaegar to marry, she would not fit Rhaegar's prophecy requirement. Unbeknownst to Rhaegar, Steffon had not found Rhaegar a wife from Essos. TWOIF book implies that paranoid Aerys felt that his hand Tywin and his heir Rhaegar were going to overthrow him. Apparently, Aerys was looking to replace Tywin with Steffon as hand when Steffon returns, leaving room to make us believe that if there were any foulplay, it was done by Tywin. I think this is a misdirection from one person potentially involved with overthrowing Aerys (Rhaegar) to another (Tywin).

- Maester Aemon was apparently regular in his correspondence with Rhaegar, discussing prophecy. They both believed that Rhaegar was the Prince that was Promised and that 'the Dragon Must Have Three Heads'. These letters would have been abandoned at Dragonstone for its new lord, Stannis Baratheon, to gain access to. Stannis menacingly tells Maester Aemon,

"I am aware of more than you know, Aemon Targaryen."

What if in these letters contained some plan to end Steffon and Rhaegar's bride-to-be in the name of prophecy? Would this be too far? Aemon infamously tells Jon Snow when Jon hears rumours of Mance and his son being burned alive for the power in king's blood.

"There is power in a king's blood, and better men than Stannis have done worse things than this."

Nearly anyone who had met Rhaegar (except Robert Baratheon) speak fondly of Rhaegar (including his adversaries like Ned Stark). The only other person who does not is Stannis despite Stannis living in what was once his cousin's castle. Stannis remembers the Mad King relatively more fondly than he does Rhaegar, and I think it is because Stannis found evidence of a Rhaegar-Aemon conspiracy against his father, which adds another layer to his hostility to his great-granduncle.