Didn't they say this game is non-canon? Raiden's change in personality from the Solid games could be them just deciding "fuck it let's make him crazy because hell yeah"
Looked into that. Apparently, it's a misinterpretation of the first comment by Kojima that's debunked in the second comment:
We've only said that the period setting is several years after MGS4. Metal Gear Rising is not part of the Metal Gear Solid series. At present, we can't say more than this.
It was obvious we had to go to Platinum, but it's still in the Metal Gear Solid timeline. The cutscenes are looked over by Kojima Productions and the script is written by us, so they should all resemble something very similar to what you'll have experienced in the past.
The fact that they are aware of this narrative disconnect though, reassures me somewhat.
According to a podcast by Kojima Productions, the apparent disconnect between Raiden's ending in Metal Gear Solid 4 and his returning to the battlefield in Metal Gear Rising will be explained.
In regards to this podcast, a video was released from the developers confirming more plot points in the game. Raiden's disconnect is explained by Kojima Productions producer Yuji Korekado:
"Raiden is part of a Private Military Corporation, he works there to provide food and a living for his family. He is providing security to some specific VIP when something happens, he's attacked by a mysterious cyborg organization, at this point he completely failed in his mission of providing security, he is completely defeated but is able to survive, his body is completely broken, resulting in Raiden receiving new armor, that's where the story takes off."