Monster.
Until Johan is saved, everything is the same. Everything changes when Dr. Tenma notices that Johan is killing an innocent man. From that moment on, the Japanese man searches for Johan (and then also others when he finds out that Johan was not the only one) to kill him. The reason is simple - Tenma sees his patient's extraordinary talent in manipulation and does not want there to be any more victims. In the case of the others, his motivation is that since they have already been subjected to the brainwashing process, there is no salvation for them. This is a pathological understanding of doctors' principle not to harm others and to limit their suffering.