First, the meaning is different.
1, harmful to someone: harmful to someone.
2. Hurt someone.
Second, the usage is different.
1, harmful to someone: harm often refers to "damage" to the body or spirit, and it can also refer to "damage" or "loss" to something [thing], causing trouble and inconvenience.
2. Harm to someone: Harm means that people's psychology, health, power or career are damaged to a lesser extent. It is a transitive verb followed by a noun or pronoun. Can be used for passive structures.
Third, the emphasis is different.
1, harmful to someone: focus on describing the harm to someone.
2. Hurt someone: Focus on doing things that hurt someone.