To spend a week on the island, and not only to be his own master for that time, but command one of the boats, pleased him very much.

It was so romantic, and so grateful to his vanity, that he was tempted to comply with the offer.

But then the scheme was full of peril.
He would "lose caste" with the Zephyrs; though, if Tim's statement was true, he was already sacrificed.

His father would punish him severely; but perhaps Tim's suggestion would be available, and he knew his mother would be so glad to see him when he returned, that she would save him from the effects of his father's anger.

His conscience assured him, too, that it would be wrong for him to engage in such a piece of treachery towards his friends; but Tim declared they were not his friends--that they meant to ruin him.
Thus he reasoned over the matter, and thus he got rid of the objections as fast as they occurred to him.