[Mad Dawgg]

"because as hand of the king he had learned of cersie's plot to kill Robert and all of his bastards, and he made the mistake of revealing his hand to her when he was not in a position of power to do anything about it."

And how did Ned learn of Robert's Bastard's?

This is Rhaegar, Viserys and Daenerys Targaryen

Notice anything?

[ConjunctionJunction]

If you are implying that Jon Snow is the son of Robert and Lyanna, then that would not make sense based on last night’s episode. Robert would have no reason to kill his own child by Lyanna, whom he loved, and Rhaegar’s Kingsguard would have no reason to be guarding the Tower of Joy.

[Future Snake Eater]

Notice anything?

Nah. Lyanna Stark had dark hair, too. Despite the Targaryens' attempts at maintaining pure blood lines, their genes could be hit-or-miss. Notice Jon Snow isn't invulnerable to fire either.

[Boogieman]

“And how did Ned learn of Robert’s Bastard’s?”

He learned of them because Littlefinger nudged him in that direction the second he got to King’s Landing.

As for your implication, Jon Snow gets the dark hair from his dark haired mother, Lyanna. Jon Snow can’t be Robert’s kid, because Robert and Lyanna never bumped uglies.