So I was doing the Dark Brotherhood quest where I was to murder Gaius Maro and put a letter in his dead body. I did all that and when i go back to talk to Gabriella and she tells me "Gaius Maro still lives, I'm surprised, I thought you were more competent". I am on PC and I don't know where to go from here. Its like a brick wall that I can't pass through. Please help <3

Kill Gaius Maro.