Achav had some merits. He was a war hero. He gave honor to the Torah and some Torah Scholars gained benefits from his property. He allowed Eliyahu to slaughter the priests of idolatry at Mount Carmel. On at least 2 occasions he showed some degree of Teshuva.
Nevertheless the main view of the rabbis in the Talmud (although there is a dissenting view) is that he does not have a share in the world to come.
http://www.ou.org/torah/tt/5765/vayechi65/navi.htmThree Kings Do Not Have a Share in the World To Come (Divrei HaYamim Bet 33:12-17, Sanhedrin 101b-102b)
In the course of discussing these verses in Chronicles that describe the teshuva of Menashe, the Talmud analyses the actions of the three most evil kings in the Tanach. Yeravam ben Nevat never repented and so forfeited his share in the World to Come. However the other two actually did teshuva and therefore their punishment is more problematical.
The very name Achav tells us that he was an 'ach' a brother who was born to be an enemy and 'av' a father to idolatry. His most trivial deed was like the sins of Yeravam, yet he refused to hand over the Torah, his most precious possession, when demanded to by Ben Hadad, king of Aram. His father Omri merited fathering four generations of kings in Israel by adding a new town Shomron, [that the Romans later called Sebastia and lies close to Shechem] whereas Achav's merit that earned him generations of kingship was his support and respect for Torah scholars.
Furthermore, when rebuked by Eliyahu for the murder of Navot and then stealing his vineyard, Achav repented and Hashem postponed the destruction of his dynasty, till after the death of his sons. So Rav Nachman said that Achav's evil deeds were actually balanced by his good ones. However, Rav Yosef objected to this since of Achav we are told "And he did evil in the sight of the Lord more than any before him" (1 Kings 16:30). This is evidenced by Jezebel his wife, who every day weighed Achav and donated his weight in gold to avoda zara. Perhaps the consequences of Navot's murder would have been even more serious so that his teshuva would not have availed to postpone the punishment were it not for the deceit by the spirit of Navot. When Hashem called for somebody to cause Achav to go to the war with Ben Hadad in which he was mortally wounded, the spirit volunteered, "I shall be a lying spirit in the mouth of all [Achav's] prophets [who will prophesy victory over Ben Hadad] " (Melachim Alef 22:21-22). Because of this, Hashem, "before whom one who lies cannot remain in His Presence" (Ps. 101:7), while allowing the spirit of Navot to draw Achav into battle, dismissed the spirit from His Presence and retained the postponement of the punishment on Achav's dynasty. However, despite his teshuva, there was something that denied Achav of the World to come. Said Rabbi Yochanan, "Achav wrote on the doors of Shomron, 'Achav has denied the G-d of Israel', so he has no share in the World to Come".
There might be an additional reason, he has no share in the World to Come (according to the Mishna), besides what he wrote on the doors of Shomron.
Bli neder, I, G-d willing, would like to comment about this on another occasion.