He only had one chance. If he messed it up, there simply wouldn’t be enough strength left in him anymore to kill Ferid. That’s why he visualized the motion in his mind and replayed it 3 times: how he would take out the needle from the pocket and drive it into Ferid’s neck.
He could do it. At least he thought he could. Ferid lost himself to sucking his blood. Or so he thought, anyway.