Manasseh is remembered as the worst king Judah ever had. So people say that Hezekiah should not have had his extra 15 years. And if you only read the story in 2 Kings there does seem to be truth in that. Yet that is not the end of Manasseh's story, although you have to go to 2 Chronicles to get "the rest of the story" as Paul Harvey would say. In fact, Manasseh was taken captive by the king of Assyria, and in his imprisonment he turned to God. He was later released and sent back to rule over Judah again. The fact that the Assyrian king let him go is in itself is amazing. What is even more amazing is that Manasseh destroyed all the idols he had built and ordered the Israelites to worship the true God and forsake idols. Can we say witness? Manasseh may have led the people to sin, but they didn't exactly go kicking and screaming. They were followers. They followed their king into sin, and they followed him back out of it.
God is in control and will grant or deny as he sees fit. We have to trust him either way, knowing he knows "the rest of the story".
(These stories start in 2 Kings 18 and 2 Chronicles 29 if you're interested in reading them yourselves.)