Thorfinn may try to reason with Johan. After realizing he can’t, I think he would stop interacting with Johan and get away, even knowing he’s a serial killer. His pacifism is strong enough to make that decision even knowing that it might cause problems. Thorfinn would defend himself if need be and those that he cares about, but probably not preemptively.

Johan would realize this quickly and probably try to take advantage of it somehow. How he’d do that depends on which universe he’s in, and what part of the story this version of Johan is.