Where is Jacopo?
http://www.youtube.com/watch?v=WbzELYKvGrI
You can kill Jacopo without starting the Memory Corridor cutscene by attacking him just outside of the interaction zone around him. Select a weapon other than fists, throwing knives, hidden blade, or poison blade; and approach Jacopo until you see the control HUD display "Interact." Now back off a little until the control HUD displays "Attack." Face Jacopo and press the attack button to kill him with your selected weapon.
You can loot Jacopo's body after you attack him (or you can pickpocket him beforehand). Taking his money is completely optional and is not needed to make Jacopo vanish.
Pick up Jacopo's body and carry him to a well. When you dump him into the well, his body will slowly sink. Eventually, the game will stop modeling his body as he sinks out of existence. Don't worry - you can still complete the mission. Just go back to the interaction zone at the Roman Theater. It's marked as the red target on the mini-map. You'll see the control HUD change to "Interact" when you are in the right spot. Press the interact button to start the assassination animation and cutscene.
Ordinarily, the game would teleport Jacopo to the interaction zone when you press the button, but his body was removed from the game when he sank into the well. Since there is no body to teleport, the game animates Ezio assassinating the air.
I thank Jack-Reacher, who was the first to report the kill animation glitch when attacking from outside the interaction zone:
http://www.thehiddenblade.com/jacopo-de-pazzi-glitch
I just tried this today and wasn't able to accomplish it. 
hi joey. in what way were you not successful? can you elaborate on what you did?
Well, I first attempted to just pick him up. I tried different angles and different lengths away from him. Nothing.
Also, I picked up a long weapon (spear) and tried to kill him with that. I even had your video up so I could mimick it precisely, and Ezio automatically killed him with the hidden blade. I noticed that they recently sent out an update before I tried it.
aw snap! theyre watching...
Jacapo's dying breathes made me extremely depressed, especially because I had to listen to them for half the video. Great job regardless, Ian, although I'm pretty sure you'll be getting an angry email soon from the elderly community.
You know what would have been a great turn in the story at this point? If instead of assassinating Jacopo, Ezio brought him back to the Villa and had someone like Claudia nurse him back to health (she doesn't do anything anyway except tell us that we made money). Because his entire family has been wiped out and the people he trusts have betrayed him, Jacopo decides to help Ezio with assassinating the remaining targets in Venice by giving him information that only he could know about (locations, intentions, planned events, etc.). It would have been nice to have an ally like that, and it would have made me feel less guilty about having to stab an old man that just got his ass whooped by Rodrigo Borgia.
Hey Ve, that storyline is yours if you want it 
You're right. It would be better if you didn't have to kill him. I wonder if there's a way to save Jacobo, kind of like a glitch way so that he's still alive when you do interact. It might be possible, but you would have to do something else besides a one hit kill with a weapon.
Haha, sounds like a fun idea. Don't think I'd write it though. I usual avoid AU (Alternate Universe) stories. I guess I have a thing for working within the canon and elaborating on theories and details mentioned in the series. I guess that's just me. 
I might write something on Jacopo though. I recently got a request to make some Templar character studies, so who knows (I'm currently working on one about Dante Moro and his wife Carlotta).
ok, how can you not feel bad after seeing the cutscene (where Borgia stabs him multiple times) and then you come along and destroy him with swords, axes, and friggin brooms!?! lol
nice vid ian.