The biggest mistake he made was the fact that he used the blood from the tip of your index finger. He sacrificed a drop of your blood.

After performing the demon summoning ritual, he thought he could use it as his slave, but since he had used your blood, now the demon was compelled to take you with it.

"Damn you demon! Go back to hell!" He shouted angrily, holding your hand. He couldn't accept that the demon wanted you to be it's wife. You were his girlfriend, not the demon's girlfriend!