Kaizen smiled with amusement as he watched how Jarnus struggled to keep up with him, and how his defenses crumbled with his every move. Jarnus was very reactive to the Psyker's smallest movements, and this speed certainly meant that he was very strong, but this quick reaction also exposed his weaknesses.
A Tank only needs to act when it is necessary to protect his allies, but a Tank should not only be someone who protects others, but also someone with leadership skills and game reading. What good is it to be a Tank who will always drag his group's frontline into the mess? Nothing, so Tanks generally have to know a lot about spatial awareness and the variations in possible scenarios.
Jarnus hadn't fully developed all that yet, and Kaizen needed no more encouragement than that to try to finish him off.
With another step, Kaizen reached Jarnus' right side and saw that the huge opening in his opponent's guard was within reach of his sword.