I am GMing a low-fantasy GURPS game, and I find that my amoral or immoral NPC's are easily seen through. On several occasions, I have tried to have an NPC dupe my players into being pawns. And every time, one particular player has caught on and turned the tables; in one case even double-crossing the guy. I don't mind my players succeeding, but in every case that its happened, I had much larger arcs planned, and they were short-circuited.

I think my problem is that I just can't get into that headspace properly. I'm not a devious, conniving, scheming person. I have a good plan for what the NPC wants and how they'll try to get it, but I struggle to improvise schemes when a PC does something I didn't expect. I am good at improvising in general, I just can't seem to do it when it involves a character like [Littlefinger](https://gameofthrones.fandom.com/wiki/Petyr_Baelish).

Do I have to just stop trying to scheme? I am open to advice about GURPS or roleplaying in general.