The direction of the feeling is a little different.
traitor
Such as a group, country, team, or friend to which one belongs.
Tell me about someone who abandoned his side.
Example: A person who sells his country to the enemy;
A person who deliberately ruins his team.
betrayer
This refers to a person who broke a specific promise, trust, or relationship.
Examples: A person who told a friend’s secret, a person who cheated on a lover.
To put it simply:
traitor = someone who betrays a group or side
betrayer = someone who betrays a personal trust or promise