Instincts. They can't be controlled by humans, and those who can control them are exceptional.
Most of the time, relying on instincts is good, as your instincts would at least want the better for you.
But relying on instincts and giving your body over to them are two different things. If you give your body to it, then the moment your body senses even a bit of danger, it will move to the path that won't have danger.
Keryu knew those things, but he didn't know that Alex did as well—a major flaw in his calculations.
In normal cases, Keryu would never have revealed the fact that he was a transmigrate, not until he was forced into a life-and-death situation. But right now, his body was being controlled by instincts, so all Alex had to do was lace his words with killing intent, making it clear that one more lie would mean death.
By doing so, he was forcing Keryu to tell the truth, and the guy did. He revealed his true name—the name he had on Earth.