What did Equiano first think the people of the slave ship planned on doing to them?

He believed he was going to be killed;

"The first object which saluted my eyes when I arrived on the coast was the sea, and a slave-ship, which was then riding at anchor, and waiting for its cargo. These filled me with astonishment, which was soon converted into terror, which I am yet at a loss to describe, nor the then feelings of my mind. When I was carried on board I was immediately handled, and tossed up, to see if I were sound, by some of the crew; and I was now persuaded that I was got into a world of bad spirits, and that they were going to kill me."