I don't remember the RJ quote you reference, but since the dagger is out and he does have it back, the taint surely can spread.

Heck, there are plenty of quotes from the books that support that.

And the Whitecloaks under his command in Shadow Rising were certainly affected, Dain certainly noticed.

So whether it's the Taint (via the dagger) or via Mordeth's influence, that attitude can spread.