There are two dimensions to Ahav's demise. On a personal level, Ahav is worse than all of his predecessors. However, the House of Omri, Ahav's royal lineage, is just another chapter in the dismal succession of kings of the Northern kingdom.
One might argue that the most severe of Ahav's crimes was idolatry, a sin on a national scale, whereas the murder of Navot was a personal crime, which didn't affect the national temper and did not influence wider ethical norms in ancient Israel. The Rambam insists, however, that murder is at the top of the pyramid, as it causes “the destruction of civilization,” the disintegration of society, undermining its cohesion and trust.
On this backdrop, Ahav's sudden teshuva is disconcerting, arousing a sense of astonishment. Can one make amends so easily? Both in the subsequent texts and in the Midrash the degree and depth of Ahav's teshuva remains an open topic.
