Does the manner of a person's death, such as dying in an accident where their body is dismembered, reflect the wickedness of their deeds or a lack of their faith?

The manner of death is not an indicator of a bad ending, unless the person died while persisting in disobedience. The Prophet, peace and blessings be upon him, considered some severe deaths, such as drowning, burning, and being crushed by a falling building, as martyrdom, which indicates that they are signs of a good ending. The death mentioned in the question is hoped for its owner to be a martyr, by analogy with one who dies from being crushed by a falling building.