Is the sudden death of a man before prayer, while he was on his way to the mosque after praising the Prophet (PBUH), an indication of a bad ending or due to his disagreement with the Imam of the mosque?

Sudden death alone does not indicate a bad ending. Rather, it indicates a good ending if the deceased was on his way to prayer. Whoever performs ablution well and then goes to the mosque is considered to be in prayer until he returns. For every step he takes, a good deed is recorded for him, and a bad deed is erased from him. There is no harm in going to a more distant mosque to achieve humility and inner focus. The imam who claimed a bad ending in this case was mistaken.