Completely disagree here. NPCs are not PCs.
I'll put a stake in the ground and say that in my (and a lot of players') opinion it is no fun to be told, "Yeah, you may know he's lying, but your character doesn't, so act like it." I get that there are some people who really get into that kind of character acting, but I think it sucks. If your players are into that, great. But if not, don't shove it down their throats.
I think the key to having an NPC adversary with a "deception" flavor is to make the deception a fun storytelling aspect of the NPCs personality, but don't make the plot depend on it. In other words, don't set things up so that if the players figure out he's lying about something, that something doesn't spoil the plot.
What I would do is overload the deception so that he's constantly lying/deceiving about everything, even mundane things (here I think of Kruschev's response to an American reporter who, frustrated that he wouldn't answer anything of substance, asked the Soviet leader what he had for breakfast. "That's classified.").
Then, the key is that sometimes this NPC is not lying. The players won't know when he is and when he isn't.