A relative is married in 1843 in St. Louis, Missouri. He is married again in Illinois in 1848. The most likely scenario is that she died. It is possible that the couple actually divorced or separated and never bothered to divorce. The divorce would have generated a court record. A separation that never resulted in divorce may not have generated any records at all.
But I should not assume the first wife died unless there is some additional evidence other than simply the subsequent marriage.