Pus is probably quite rich in protein, the remains of bacterial cells walls might elicit an immune response and may contain other molecules such immunoglobulins and interferon so eating pus might be a trait that had an evolutionary value in producing the modern human person. As such it would not be a thing that you need feel bad about yourself over - you have no choice, evolution never rests.

Question is though, where do you go when you run out of your own pustules? Please don't answer that.