Ask the majority of Jim's employees to describe Sebastian Moran, and, depending on whether Jim or Sebastian were around, they would reply with words like 'favourite', 'right hand man', 'indispensable'.
Jim wouldn't admit this out loud, but 'indispensable' was precisely what Sebastian had become. He was one of the few who remained each time Jim 'thinned the ranks' so to speak. The only one who he wouldn't consider getting rid of, because the thought never crossed his mind.
He'd come to depend on the sniper for a lot more than ensuring that his hits get taken care of efficiently. The man was one of two people who had seen him at his worst; he'd been there when Jim's need for distraction grew so great it overpowered his sense of survival. When even staying alive became a chore.
On those nights, Sebastian would recieve a phone call from an unintelligable Jim, who, voice slurred and shaking, would mumble the same two words over and over again. G'bye 'Bastian. It would then be a race for Seb to get home before Jim could do something stupid; all the while trying to keep him talking so he knew the man was still alive.

It seemed like just a normal night when it struck. Jim didn't know what exactly caused it, but one moment he was sitting at his desk humming along to some classical music piece that took his fancy at the time, and the next he'd snapped and had thrown the book he was reading across the room. A torrent of emotions seemed to flood his system; anger, pain, sadness, built up over the many years he had spent trying to lock it all up inside. His head pounded, and he started hyperventilating. All he could think was 'I need a distraction. Anything.'
Which was why, 10 minutes later, he was standing in their shared flat with a concoction of pills in one hand, and a glass of water in the other. Without hesitation he swallowed the handful of drugs and stood there, waiting for the pills to take effect. While he waited, Jim lazily looked around their flat, noting how bland everything was. So plain. So boring. Maybe he'd do something to change that, later.
Jim wandered into the bedroom, heading towards the draw he knew Sebastian kept a firearm in. Opening the draw, he picked up its contents and drew it out; watching the light glint off the barrel. He held it up to his head and made a pretend 'pew' noise, before collapsing onto the bed behind him. He giggled, watching as his world started to blur around the edges and hearing his heart thunder in his ears. His limbs felt heavy, and he almost wanted to succumb to the freedom the darkness promised. But first, he had to do something. With some difficulty, he extracted his phone from his pocket and clumsily pressed the speed dial for Sebastian. Holding his phone up to his ear, he heard the dial tone before a faint 'Hello?' reached his ears. He let out a bark of laughter, before slurring " 'Bastian! 'Bastian, I found one of your little toys." He waved the gun around above his head as he spoke, fingers playing with the trigger in his hands. He was dimly aware of Sebastian asking where he was, and he replied with a lilt to his voice, "Where do you thiii~iink I am, doofus? Waiting for you to come home. Though don't be too~ooo long. I might get bored, and my finger might slip on the trigger…" He giggled again, holding the phone up to the gun so Sebastian could hear the sound of the safety being taken off. It obviously provoked a reaction, because Jim heard a tense 'Fuck!', before the sounds of feet thudding against the pavement echoed through the phone. He bought it back to his ear, whispering "Run run as fast as you can, you can't catch me I'm the gingerbread man," before the phone slipped out of his hand onto the bed beside his ear.

He must have blacked out for a second, because the next thing he knew he was being shaken awake by a pale-faced Sebastian, who was shouting obscenities at the man lying on the bed. As soon as he saw Jim was coming round, he hissed a quick 'Thank fuck' before hauling him to his feet and into the bathroom.
With the practiced ease of someone who has done the same thing multiple times before, he pushed his fingers down Jim's throat, moving out the way quickly when it resulting in him violently throwing up into the toilet.
Once he had finished emptying his stomach of the multitude of drugs, Jim sat back against the toilet, exhaustion taking over. He was dimly aware of Sebastian sitting next to him, and he mumbled, "Careful of the shoes, 'Bastian. They're Westwood." He heard Sebastian chuckle beside him, and went to look over at the sniper, but instead got distracted by a shiny black object on the floor beside him. He picked it up, before motioning to Sebastian and saying, "Y'dropped your gun, Sebby. Gotta keep a closer eye on them y'do." He must have taken it off Jim when he dragged him to the bathroom, but abandoned it in favour of getting the drugs out his system. Sebastian went to take it off him, but Jim, with surprising agility, yanked it out of his reach. He unsteadily got to his feet, hitting Sebastian on the shoulder before lurching out the door towards the living room, his exit accompanied by a sing-song 'You're it~!'

Jim heard the sounds of pursuit, and manic laughter clawed its way out of his mouth; eyes wild as he looked for a place to hide. But before he could, he saw Sebastian reaching for him out of the corner of his eye, and he twirled on the spot, pointing the gun at the sniper. "Stay where you are!" he screamed, his amusement vanishing. He was coming down off his drug-high, and that meant his mood was fluctuating constantly. He was aware that Sebsatian had frozen in position, the gun mere inches from his chest. "I could blow your heart out right now if I wanted," Jim muttered, eyes fixated on the point of Sebastian's chest the gun was pointed at. "Paint the walls with your blood… it'd certainly liven the place up a bit." He stepped closer and closer to Sebastian, until the tip of the gun was pressed up against his chest. Jim could feel his heart through the pressure of the gun, and smiled to himself. "Fear. It affects even the ones who hunt in the night. We should be above fear, Sebastian. But no one can truly escape their emotions."
Jim released the pressure on Sebastian's chest a little, and that's when he decided to make his move. Sebastian's arm lunged for the gun and Jim stepped backwards to try and avoid the incoming collision.
It all happened so quickly after that.
One moment Jim was upright, the next his foot had connected with something behind him and he'd lost his balance, falling to the floor. His grip on the gun slipped, and as he went to put his hand out to lessen the impact of the fall he pulled the trigger. There was a loud BANG!, and seconds later Jim looked up from where he had fallen to see Sebastian lying on the floor opposite, a huge hole carved in his head by the bullet.
Jim quickly dragged himself over to the body, head clearing up fast as the adrenaline started flowing. He shook Sebastian's shoulder trying to get a reponse. "Sebastian? Sebastian, stop fooling around. This isn't funny. Look, I'm sorry I took your gun. Here, you can have it back." He pressed the gun into Sebastian's now lifeless hand, wrapping his fingers around it. "Take it, Sebastian. Take it." As he gained no response, Jim grew angry, and hit Sebastian on the arm. "If you don't fucking take the gun now, I swear to God I'll make you wish you'd never been born." Still no response. "I mean it Sebastian! TAKE THE FUCKING GUN NOW." Jim was on the verge of hysterics now, shaking Sebastian's frame violently. "SEBASTIAN. I DON'T WANT TO PLAY THIS GAME ANYMORE. YOU'VE WON. Sebastian? Seb- Seb please… Seb, please don't leave me. You can't. I don't want to fucking be alone again. Who else have I got? Sherlock? He's going to be gone soon too… and then I won't have anyone. Seb, please… please…" He looked up; vision going blurry with unshed tears. He wasn't even sure why he was crying. He never cried. It must be the exhaustion, and the leftover drugs. He vagely noted the red splatters covering the walls, and thought to himself How can I see beauty in this?
Giving into his exhaustion, Jim closed his eyes. He stayed in that position all night; head bowed, shoulders slumped, blood splattered on skin. And the fingers of one hand wrapped around Sebastian's keeping the gun in place.

Two weeks later, and the body of Jim Moriarty was found on the rooftop of St. Bart's hospital. Sherlock would always remember the man's expression just before he took his own life. Deep beneath those dead, black eyes, he saw something. Something that reminded him the man standing in front of him was still human. Sherlock saw happiness.