When you infiltrate Palazzo della Seta, a cutscene starts before the assassination. If Emilio gets spooked before the scene ends, killing him results in no memory corridor. In addition you get a clear view of the scripted "arrow storm" from the thieves killing all the guards inside the palace. Here are three ways to frighten Emilio during the scene:
1. Smoke bomb. To ensure the target notices the smoke bomb, don't stand too close to the edge of the roof and face the inside.
2. Get seen by the target. To do this I first used an exploit to remove the front door and get inside. https://youtu.be/ZvPCx2sIYRg
3. Dead body. Throw a guard form the roof to where Emilio will see him during the scene.
Scene Glitch
If Emilio doesn't see the dead body, another guard an agile notices him. This results in the scene unable to end, the camera stuck at the end of the scene. If you pause the game by bringing up the start menu and unpause, the camera goes back to the start of the scene.