I didn't read the link, but you have a fundamental misunderstanding of assassination markets.

  I bet $1B that Julius Caesar will not be killed on March 13 with nightshade.
  I bet $1B that Julius Caesar will not be killed on March 13 with hemlock.
  ...
  I bet $1B that Julius Caesar will not be killed on March 15 by being stabbed to death in the back.
  ...
  I bet $1B that Julius Caesar will not be killed on March 17 with nightshade.
  I bet $1B that Julius Caesar will not be killed on March 17 with hemlock.
  ...
  1,000,000 automatically generated bets omitted.

Sure, people can bet some cash and attempt to get a cut of the assassin's money. Traditionally, you put that cash back into the pot, so (for all but the successful event) it goes to the assassin's pockets.

The first-order issue is that the assassin needs to bet a bunch of cash to out-bet the zero-information speculators. Some Roman trillionaire could bet $100K that the assassination would happen for each slot, drowning out the cash of the actual assassin. Of course, this would cost them $100B if placing bets are free, and there are a million scenarios.

The next big problem arises if people can watch for movements on a given position in real time. The market can fix that by running the feed on, say, a 1 hour delay. So, while they're sharpening their knives at the Senate, the Roman congress-critters can each put in a bet via cell phone.

Of course, then the (totally ethical, I'm sure) people running the assassination market could siphon money off by spying on the realtime feed. This tertiary problem is solved by making sure those people are generally well-known, giving them an incentive to not piss off assassins.