The two questions you should ask are:
whether it's the player or the character that's uncomfortable with your proposed shift in mood, and
if it's the latter, whether your players would be OK with the kind of intra-party conflict the shift would generate.
Is the player really OK with it?
If your player is uncomfortable with the campaign turning evil (or at least morally ambiguous) and just "doesn't want to go there", then I'd generally suggest not going there. You started the game with characters that are noble heroes, and all your players seem at least OK with that — you shouldn't change that in the middle of the campaign, unless you're sure your players will be OK with the new theme, too.
If you do still want to try the mood shift, please do make sure to talk things out clearly with the objecting player. Even if they say that it's their character who wouldn't go along with an evil plot, they could be using that as a cover for personal unwillingness to explore such themes. Just make it clear — preferably in private — that they can say "no" if they want, and that you'll respect it.
(Also, no discussion of game themes and intra-group tensions would be complete without a link to the Same Page Tool, so here it is.)
So the player is OK with it, but their character isn't. Now what?
So you have five nominally good characters who are increasingly tempted to "cut corners" and use questionable means to achieve their ends, and one who refuses to do anything of the sort. That's a great recipe for lots of tension between the party members.
This can be a good thing or a bad thing. It can be bad because it requires a tricky balancing act between party cohesion and inter-character conflict, and maintaining that balance can require some skill from both the GM and the players.
On the other hand, it can be good because, done well, it can be really fun to play and makes for great plots.
Note that such conflict doesn't necessarily have to turn into an actual PvP fight (which, based on what I've heard, would generally be a bad idea in 4e anyway). In fact, I'd suggest that, at least 99% of the time, this is something you shouldn't let happen, just like you generally wouldn't let the party get all killed by falling rocks or let the characters just split up and go their separate ways. It just doesn't make for a fun game.
Instead, if you want to try this, you and your players should agree in advance, out of game, to never let things get quite so far that a fight (or a party split) would be unavoidable. This means that the other players — and their characters — need to stay aware of the fact that they need the "pure" character's help (for some reason; preferably a plausible one) and that they therefore have to stop just short of doing anything that this character would find totally unacceptable.
It also means that the player playing that character has to be OK with the fact that their character also, for some reason, needs to cooperate with the others, even if he (the character, that is!) is feeling distinctly uncomfortable about it.
This does require a certain amount of "meta-game" communication between players, and possibly some creative adjustment of character behavior or even plot events to avoid escalating the tension too far. Do tell your players that it's perfectly OK to, say, ask another player out-of-game how their character would react to a particular action by another character, or even to just raise a hand and say "Guys, please don't do that, there's no way my character wouldn't righteously smite you if you did it."