2 Kings 21:23 (ESV)

23 And the servants of Amon conspired against him and put the king to death in his house.

Amon dies as he is killed by his servants. The reason for killing him is not mentioned. There can be several reasons for his servants to do this, such as, not being pleased with his foreign policy, not being pleased with his domestic decisions, or perhaps one of his servants wanted to be on the throne—which had happened a few times in Israel.