You’re running in a race. You’re ahead. But, in order to keep your lead you must keep going strong. If you let up even once, you risk losing or falling behind. Think of your villain as running in a type of race. It’s the kind of race where he/she is coming at your main character (MC) and not letting up (remember, this is their job). So much so, that your MC gets kicked back down before they get even half way back up. Your villain is on a roll. They can’t stop because, if they do, they run the risk of losing and the MC overtaking them.
Think of many different ways for your villain to trip up your MC. Think outside the box. I am currently reading a set of books by Jeff Carson. They’re David Wolf mysteries. OH MY GOSH!!! Talk about heart pounding suspense. The villain(s) never let up. Often times, while in the middle of one of these books, I couldn’t stand the suspense any longer, so I went to Amazon and read the summary of the next book. Doing this reassured me things would turn out ok in the current book I was reading. Then I would start the next book right after the one I finished and go through the cycle all over again.
What caused this mind blowing suspense? The villain(s). AND, in most cases there were more than one. All from multiple subplots and all out to get the MC. If you want a great example of a true villain(s), read those books. You can find them on Amazon. See the link below for book 1:
Foreign Deceit by Jeff Carson