Cell claimed he would admit defeat and leave everyone alone. But Cell was also a big liar, he didn't want to fight all the Z team at once when several of them proved close matches. So he played along hoping they would keep their word.
As he defeated them one by one, it became more and more obvious he had no intention of keeping his word, hence the reason Goku finally died. Thank Goodness Gohan has that temper of his. Too bad he became such a weakling in later episodes.