If I beat the Wall of Flesh does it place ALL worlds I have into hardmode?
Does defeating the Wall of Flesh make every world I've created go into Hardmode?
If I create a world after defeating the WOF, will it be created in Hardmode?
No. Hardmode is per world. When hardmode starts after you defeat the Wall of Flesh, it will only affect the current world, and doesn't affect any others you may have saved or created.