There have been many very interesting and yet tragic studies of Jewish holocaust victims, and there is a whole syndrome of psychological responses to knowledge that you are inevitably going to die. Over and over again, the holocaust victims went to extraordinary lengths to just find some way of leaving a record—some kind of a track, even if it was just a little paper written and rolled up and stuck into a crack in a cell, or a diary. They desired to let it be known that they suffered that. It is called the survivor witness. Or the will to survive through witnessing. Abinadi was certainly going to die, and Alma may have wondered if he would be next. Alma, then, was driven by the need to leave a record. Mormon, at the very end of the Book of Mormon, when he knew that his people were done for, he said, "All I can do is stand as an idle witness, but I will leave a record." Alma may have been driven by the same motive.
Book of Mormon Central, "How Can the Book of Mormon Survivors Give Us Hope? (Mormon 8:3)," KnoWhy 393 (December 26, 2017).