Thrummingbirds serve to fan the flames of existing Phyrexian infection. They swoop in, carrying transmittable cocktails of mutating magic, causing creatures affected by phyresis to move to the next stage of "development." While certainly part of an evil civilization, thrummingbirds are not themselves truly evil, as they are mindless, birdlike Horrors, merely fulfilling the purpose for which they were created. If you see a Thrummingbird, you are not facing Phyrexia's true madness; your best hope is to swat the droning pest before it plies its contagious trade.