I have one doubt about Bhabha scattering which is scattering between one electron and one positron, two oppositely charged particles. Why should they scatter each other away rather than attract and eventually annihilate themselves, giving rise to the pair annihilation process? (This process is dealth with in for example, Peskin-Scroeder's book on QFT). But I do not understand when they would annihilate and when they would scatter each other. Both are described in QED, by exchange of photon. But in one case there is attraction in the other, there is repulsion. How is that possible?