I HATE Nash's re-design / character change. The dude was focused on his mission on taking down Shadowloo but he was still kind, especially to Chun-Li. Now he's just a Emo Git who has a Sasuke complex where he has to get revenge on Bison and NO-ONE can stand in my way, NARUTO~! God.... it makes me want to punch Nash for being a dick to Chun. "I'll kill you were you stand" what bullshit.
I personally think it's an interesting inversion of his personality. He put everything into stopping Bison for justice, but, it turns out that
his own people in the US government were bought off by Bison ages ago.
They fed him tons of bat intel, diverted his path, but still that wasn't enough to stop him. Things only got worse for them when he and Chun Li partnered up and while posing as a couple for a time, were able to really sniff out leads. So eventually they had Charlie eliminated and aided Bison in his "murder".
He's hell bent on finishing Bison for good since he sees the damage he can do and how much has been done since he's been gone. I'd be "emo" too. He did it the other way at first and it got him nowhere, so it's not right, but understandable why he's gone extremist; he tried with friends and allies, he had humanity and even a sense of humour, but all he got was shot in the back, literally. We've been discussing this on SRK, but it looks like his Alpha 2 ending is his canon fate now, and SFA3 and SFII were merged by way of the anime clips released around SF4.
I don't know what his story is in SF5, but all the heavy hints all but point to Gill, Urien and the Illuminati. I theorize that they found him near death, and offered him the power he would need to launch his crusade, but he left before he could be fully finished. Interesting note, he's, in my theory, operating under free will. Gill may be a lunatic, but he's not Bison, and he doesn't brainwash.
Aesthetically I find his design interesting. He paid a tragic price for his crusade and his betrayal and he's marching on wearing his scars, not caring what he looks like. He also is wearing the 3rd Eye opal that Gill and Urien wear. That's got to be significant since we don't see the Ko-lin (Gill's beloved secretary and possible lover), underlings or G-Project creations wearing such a thing.
Personality wise, it makes sense. He knows the depths of Bison more than anyone in the cast and he feels that Bison needs to be taken down no matter what -- he's always getting away or throwing a smokescreen, so Charlie's basically like "Look, I care about you people, but there is no more time, too many people have died or been corrupted. IF you try and reason or talk me down, that's valuable time that's being wasted. Time wasted means more horror Bison can inflict. So if you keep me busy, I'll kill you so that I'm not slowed down."
The interesting thing is that he's not as extreme as he's presenting. His intro basically says "stop me and I'll kill you" yet if he wins such fight, he takes mercy on Chun Li and gives her another warning.
I can see his future (if he doesn't get killed off for real) resembling Big Boss' path. Bison is but a symptom of the problem. His own people betrayed him, there's corruption and evil in the world. He may try to go the Rolento route, build up his own forces and try to "make things right" by force. Guile and Chun li will clash with him along the way, but since he's not bainwashed, there's always potential hope that he can be convinced to return to a kinder, more forgiving path.