In Lord of the Flies, Piggy explains to Ralph that Jack can be very dangerous - not to Ralph, but to whom?
Here's the bit from the novel you need:
“I’m scared of him,” said Piggy, “and that’s why I know him. If you’re scared of someone you hate him but you can’t stop thinking about him...I tell you what. He hates you too, Ralph—"
“Me? Why me?”
“I dunno. You got him over the fire; an’ you’re chief an’ he isn’t.”
Piggy has sensed the discontent in Jack. Jack needs to be in charge, needs, to dominate, needs to hurt. And Piggy is scared. The real answer to your question is in this quote, though, from the same section of the novel:
"He can’t hurt you: but if you stand out of the way he’d hurt the next thing. And that’s me.”
Piggy knows that he is vulnerable to Jack - he can't physically stand up to him, and he isn't confident enough to challenge him outside of a meeting with the conch in his hands. And, true to form, Jack does indeed come to hurt Piggy.
Perhaps, though Piggy's real insight into Jack's dangerousness comes with a genuine foresight as to what Jack would be like as chief:
Piggy gripped Ralph’s arm.
“If Jack was chief he’d have all hunting and no fire. We’d be here till we died.”
This is, of course, almost what happens: Jack doesn't keep the signal fire going when he takes over control of the island. For all Jack's instinctive leadership, his real danger perhaps lies in his inability to plan ahead.