But when he was strong, his heart was lifted up to his destruction: for he transgressed against the LORD his God, and went into the temple of the LORD to burn incense upon the altar of incense. He transgressed against the Lord - "He sinned against the Word of the Lord his God." - T.
Went into the temple to burn incense - Thus assuming to himself the priest's office. See this whole transaction explained in the notes on 2 Kings 15:5 (note).