So he now had no idea why he fights, his only reason because that's the only way to live that he can manage. He then realizes that what he's doing isn't useful at all for a new criminal group always replaces the previous one. So starting off, after recent events of his life, he decided to be a vigilante and take down all criminal groups he comes by. I'm going to begin by telling a bit of background about Sam, and then his DLC, and then the main part of the game.

His true nature was awakened later in the game and he took down World Marshall. When N'mani was kidnapped, after taking down the Ray he goes after Sundowner, which then kills and leaves Jack to Sam.ДЄnd we all know what happened after that. Maverick investigated the matter and thus let Jack do the job. He wanted to have a war to 'cleanse' the false image of freedom that the people see, to 'jumpstart the economy', to 'End pointless wars', to make war 'a business', most of which I see is true, he wanted to do those, but his methods are wrong. And no one else but Armstrong is behind this. A sudden attack at Africa, exactly where the Prime Minister is, to drive attention to war. He joined Maverick to help rebuild the damage done during the war.

If you review previous Metal Gear footage, after the fall of the Patriots, and after Snake defeated Liquid (Which was one of the best boss fights tbh), Jack just wanted to live with Rose and his son, just like nothing happened (He also sort of stated this before the Monsoon battle). MGR shows us the reawakening of Jack's true nature, what he was he grew up, a cold blooded killer. Jack's part in MGR is pretty straight forward. Ill begin by talking about Raiden's role. Credit goes to AnamGGSS for explaining this and blowing my mind in the process

Metal gear rising ost main theme