“But Don John merely represents a more general mistrust in the play – he is not its sole source. After all, his is a tiny part (no sniggering at the back): he has only 4 per cent of the play’s lines. He does, however, symbolize something larger than himself. And perhaps this is why he is given the identity of bastard. His own malevolent illegitimacy might be thought a kind of proof that women can – and some do – sleep with men not their husbands. Don John the bastard is himself the very certification to stabilize the play’s paranoia about women’s faithlessness. His status as a bastard thus confirms the play’s worst fears.”
Book:This Is Shakespeare