Why does the Lord of the Flies say "I'm part of you" to Simon?

The Lord of the Flies says, "I'm part of you" because Simon understands that evil is intrinsic to the nature of man.

Shortly before Jack's tribe raids the beach and steals some of the burning branches from the fire built there, Simon slips away to the clearing in the glade where he has meditated before and reveled in the beauty of nature. But, now in this clearing he finds a bloodied sow's head impaled upon a stake that the hunters have left as an offering to the beast.

The half-shut eyes were dim with the infinite cynicism of adult life. They assured Simon that everything was a bad business.

Unconsciously, Simon answers, "I know that." Simon looks up and gazes at the sky, then he lowers his head, shutting his eyes, even shielding them from the sun.  At this moment for him, reality seems "illusive and without definition." When he again looks, he sees the eyes and teeth and blood, and "his gaze was held by that ancient, inescapable recognition."

Simon finds it difficult to look away from the Lord of the Flies; further, it seems to talk to him, asking why he is alone. Then, it asks,

"You knew, didn't you? I'm part of you?....I'm the reason why it's no go? Why things are what they are?"

Simon does know; in fact, he is the only one who understands the beast and evil because he confronts it.