His participation in this resistance is a logical consequence of his background. Everyone knows the man as a doctor and healer. I doubt whether people also know him as a soldier. In the photo below we see him as a soldier in 1934.
The description reads:
Portrait of surgeon J. Roeper in military captain's uniform, lived at Oude Koemarkt 6
Max Jacques Roeper Bron: Museum Heerenveen
And so his participation in the doctors' resistance can easily be explained. If you are a captain then you are the commander. You have a thorough military education and that background, I think, also makes you more inclined to chauvinism.
So the step towards active resistance against the occupier is only logical.
The man was (ex) military and therefore familiar with weapons and combat techniques.
So from the point of view of the occupier, the necessary danger had to be feared from this man. If your goal as an occupier is to exterminate all Jews, that of course starts with breaking the resistance. So a Jew who could skillfully handle weapons and also could lead men. He actually had to be killed as quickly as possible.
I deduce from his rank that the man could lead men! I don't know much about military ranks, but I think a captain is quite a high-ranking officer.
Source: Museum Heerenveen