It was her mission. It had to be finish or The Boss wouldn't be in the KGB or anything else. It had to be finish.
The Boss killed The Sorrow because it was part of her mission. She had to do it because if she failed her mission then Metal Gear Solid 3 would change than it is today.
A Metal Gear is a bipedal nuclear tank which features in most of the Metal Gear games. The first made Metal Gear is Metal Gear Raxa which was known as an ICBMG, then there was Metal Gear TX-55 and Metal Gear D. The next Metal Gears were Metal Gear Rex and Metal Gear Ray. After those they finally perfected the Metal Gears and made what is codenamed Metal Gear Gekko.
You cannot kill The Sorrow in Metal Gear Solid 3: Snake Eater as he is already dead. To end the boss battle, you must walk to the end of the river while avoiding the ghosts of the people you have killed. When you touch The Sorrow's skeleton, you will die. At this point, you must open the Item menu and select the Revival Pill, which will end the battle.
If you mean story wise then I believe it goes: Metal Gear Solid 3: Snake Eater Metal Gear Metal Gear 2: Solid Snake Metal Gear Solid Metal Gear Solid 2 Metal Gear Solid 4 But I may be wrong, and that isn't counting either of the Portable Ops games.
There is a ghost named the Sorrow. He shows Snake the ghosts of everybody he's ever killed in a dream-like scene.
metal gear solid1, metal gear solid 2 sons of liberty, metal gear solid 3 snake eater, and metal gear solid 4 guns of the patriots.
Metal Gear Solid 4: Guns of the Patriots
There has been no confirmation of a Metal Gear and Metal Gear 2 remake. They are playable on Metal Gear Solid HD Collection in the Metal Gear Solid 3 portion, but thats the only thing that would be considered a remake right now.
Metal Gear Solid, Metal Gear Solid 2: Sons of Liberty and Metal Gear Solid 3: Snake Eater
the metal wears down from the gear turning metal against metal
Metal Gear Arcade happened in 2010.
Metal Gear Acid happened in 2004.