He had been working at Moultrie, then decided Sumter was the better place to be, then he was getting the message that things might turn hostile and his friend may be the one aiming at him. In the meantime, he was running low on supplies and trying to figure out what ships were where and when they'd do something to help him out.
The arsenal was in the hands of the South Carolina Militia too. Guess he felt like those walls were closing in. Wonder if he'd have changed sides if he knew of that little secret message ol Abe sent Porter? I would not blame him.