Humming happily, Simon left Jayne to recover from the paralytic and ponder on his words. River would keep an eye on him, and probably terrify him half out of his mind while she was at it, but she wouldn't actually hurt him, not while he was helpless. And even after Jayne's natural terror of River had been blunted by time and familiarity, as it was sure too be, eventually, Simon's words would still be in his mind, reminding him of the consequences of his actions, far more threatening than any physical threat on Simon's part could be.
And the ironic part was that Simon had honestly meant every single word that he had said to Jayne. He would never hurt a patient under his care, no matter what they had done too him. He had taken a sacred oath, after all.
Jayne would never be able to truly believe that, though, because, had their situations been reversed, Jayne would probably have taken his revenge on Simon at the first possible opportunity. And now that Simon had pointed out Jayne's unavoidable vulnerability to him, he would never be able to forget it. It would always be there, in the back of his mind, whispering that, someday, he would be unable to avoid being at Simon's complete mercy, and did he really want his Doctor to be angry with him when that happened? Jayne's own mind, well-used to analyzing all the possible threats in a situation, would do a far better job of tormenting the man than Simon could ever hope to.
Hopefully, it would be just enough to make the man think twice before betraying them again, but not enough to terrify him past all reason. He didn't need to start worrying about Jayne trying to kill him in his sleep, after all.
Revenge was indeed a dish best served cold!