How? How could they have killed both of them without all that bloodshed? And how do you know that would have ended the bloodshed? Given what we've seen so far, it would just lead to more succession wars. Not to mention that both Cersei and Dany had armies and we know for a fact that Dany's army would have fought to the death to avenge her.
When in the history of Westeros has shit ever worked that easily. Robert got killed by a boar and that shit led to a war between 5 different factions. If you read the books, you know every time rulers have just up and died, it's always resulted in huge wars. There was no way this ended without lots of people dying. If you don't see that, you are basically ignoring every bit of history they've showed you about Westeros.