Does anything prevent the president from ordering the murders of his enemies and promising to pardon anyone who gets caught?
Considering the outcome of the January 6 prosecutions, probably not. That wasn't an explicit
quid pro quo, but the promise is effectively out there.
The boundaries of the current ruling haven't been tested and have no effective precedent. The Chief Justice essentially invented this doctrine out of thin air, so no one but he knows what was going on in his head or how this is supposed to work. If the
quo is a pardon (or any other core function), then any discussion of a
quid in connection with it is immune from any kind of prosecutorial inquiry. Where we think this might run aground is whether the proposed deal is public knowledge (e.g., the President announces it from the Rose Garden), and whether the
quid is separately actionable regardless of any
quo that might follow. But if the President pulls someone into the Oval Office and says, "If you kill So-and-So for me, I will pardon you for it," then the current thinking is that no prosecutor can touch that.
The current SCOTUS doctrine is that only express
quid pro quo corruption is punishable in any context. You can't prosecute according to a general understanding—no matter how obvious—that anyone who commits murder to advance the President's objectives will be pardoned.
ETA: I need to disclaim part of this by noting that there is famously no federal statute for murder per se. You cannot be charged for simple murder under federal law, so you cannot be pardoned by the President for it. But there are a number of muder-adjacent crimes (e.g., terrorism) you can be charged with under federal law, including some that carry a death penalty.