We can consider the beams to be current carrying wires. The proton beam is going from north to south so the current for it will be directed from north to south. And the electron beam is moving from South to north so the current will be directed opposite to the direction of the beam, i.e. from north to south. So we get the case of two current carrying wires carrying current in the same direction and they attract each other. So the electron beam will be deflected towards the proton beam.