God was ready to support Jeroboam, but he needed to obey God. Unfortunately, he thought he could strengthen his power more by building a temple in his own territory rather than have the people go to Jerusalem. In Deuteronomy 12:5, God had commanded He only be worshipped in the place He chose. God also punishes His prophet for being deceived into disobeying.
These acts of disobedience may seem minor and God’s punishment harsh, but God expects to be obeyed in all cases.