Ubisoft owns all except Leonardo going mental.

"Hello, Ezio- go away," is a reference to Shaun saying, "Hello, Desmond- go away." (Va bene also means okay in Italian. Molto bene means very good.)

"Merda!" came a cry behind Ezio. He turned to see Leonardo staring with wide, terrified eyes at a tangle of metal and material on the floor. He dropped to his knees, practically tearing his hair out.
"How did this happen?" he began, then saw what had snagged on the contraption, which had made it fall and break on the floor. It was Ezio's cloak that had caught. It was Ezio's fault.
He stood abruptly, wild anger flashing in his eyes. Ezio took a step back warily. It seemed that the man who claimed he was a pacifist, a person who wasn't violent, was now going to change that.
"Do you have any idea how long it has taken me to design and build this?" he demanded in a choked whisper that worried Ezio more than if he'd been shouting. His voice rose in volume when there was no reply. "Nine years. Nine whole years! And you break it in a matter of seconds! Buffone! Idiota! Get out of here! I never want to see your face again!"
A lamp shattered on the door next to Ezio's head as he left the door. Leonardo wasted no time in slamming the door unceremoniously on the startled Assassin. He stood there, gaping blankly. He hadn't meant to, and Leonardo had never been so angry before. When his flight test failed he had flown into a fit of anger, but that was nothing compared to this rage.
Ezio stood outside, waiting for Leonardo to show himself so the Assassin could apologise. There was no such luck.
He knocked softly and called the man's name.
"Didn't you hear me? I never want to see you again!" came the muffled shout from within. He heard a series of crashed and things shattering. Leonardo was in such despair he was damaging all his other works. Ezio winced at the sound of tearing canvas. That could have been La Gioconda, the Mona Lisa painting he had been working on. That was when he had admitted how pretty girls would provide little distraction.
Ezio cleared his throat at this memory, and attempted to open the door. It was suddenly wrenched open and he was met with the most furious face he had probably ever seen. Leonardo's eyes glinted with insanity, and his hat was askew.
"Hello, Ezio- go away," he snarled, and yet again the door slammed. Ezio was at the end of his tether. Leonardo had worked on the invention for nine years and Ezio had just broken it. But it was an accident! Leonardo was overreacting; it wasn't as if Ezio had done it on purpose. His face twisted into a frown.
He barged the door open, stumbling inside, and Leonardo spun with a cry of anger at the sight of the Assassin.
"How dare you enter-" he began. Ezio flicked out a hidden blade and positioned it at the man's throat in quick succession. Leonardo swallowed his words, blanching. But he still looked at Ezio with slightly crazed loathing.
"Listen," Ezio snapped, nicking Leonardo's throat threateningly, "I didn't do it on purpose. I'm sorry for damaging whatever it was, but you can't go destroying your life's work in a fit of rage. Va bene?"
"Va bene," Leonardo mumbled. When the blade was removed he sank to the floor. Only now did he seem to register the destruction and havoc he had wreaked upon his workshop. He took his face in his hands, not with pity for himself but pity for his customers. His work, their property-to-be, was ruined.
Ezio was relieved to see that the Mona Lisa had escaped unscathed. The ripping canvas was a plain one he hadn't started on, and frustration had made him despise the blank space.
"What have I done?" he whispered. Ezio crouched by the distraught man, placing a hand on his shoulder.
"Calm, Leonardo. We can fix this," he reassured. The man looked up, his eyes watery with distress, his face completely dejected. Ezio could tell he hated himself for what he'd done.
"First, calm down," he murmured. He waited until the man had stopped trembling with horror at his misdeeds. "Molto bene."
"Now what?" Leonardo asked, twisting to look at all the damaged contraptions. His breath tightened at the sight of the fire was burning away happily, fed substantially with smouldering, crumpled ideas. Ezio, the gesture not meant to imply anything other than brotherly reassurance, took the man's face in his hands so Leonardo could concentrate and calm down again.
Ezio helped the man stand, and he immediately went to get a glass of wine to rid himself of his nerves.
"Now we'll see what we can do," Ezio continued, carefully taking the glass so Leonardo couldn't drink anymore. Being drunk wouldn't help much.
He began to set things back into place, straightening this, adjusting that until the workshop was respectable again. Finally Leonardo seemed satisfied and happy again: he was his old self, smiling and cheerful. He kept apologising profoundly for what he had done and what trouble he had caused the Assassin. But Ezio brushed the apologies off, blaming himself for the initial start of the dilemma by breaking the contraption.
Eventually, Leonardo was completely back to normal. Now, when thinking of it, he laughed at what had occurred, shaking his head sympathetically at the pezzo di merda.
He swung his arm Ezio's shoulders, admiring the change the Assassin had made to the messy shop. Granted, it was still untidy, but it was now organised chaos, not just ... mess.
He grinned at Ezio, gesturing with his left hand, which was by Ezio's head.
"I think I'll be very happy indeed," he said with some enigmatic emotion. Ezio smiled back, glad Leonardo was back to normal.
And he hoped that would never happen again.

[A.N.: Most Ezio/Leo fans would be flailing right now, anticipating fluff and more stuff. Sorry to disappoint you, but I don't type yaoi romance - and definitely not lemons! I prefer friendship and, at most, bromance. Just not romance. Because: 1) That's not me - I'm not homophobic, I just don't type it, and 2) my sister would probably murder me. Or just never speak to me again. So ... um ... yeah. Thanks for reading! R&R!]