The classification of a forfeiter of land as a rodef states that "If a Jew gives up the land of other Jews to Goyim, and he persists in this, that is, he gives up the land of three or more Jews, he is a Rodef."
Where did the writer get the number 3? Its even 1 and its Din Mosser (someone who hands over Jews or Jewish property).
Also Din Rodef has to do with the fact that he gave guns to the Arabs who did and would use the guns against Jews and also giving them opportunity to kill more Jews. That is why Din Rodef.