Bill helped him when he needed to overthrow the current leader, who is a bully.
The word bully isn't commonly used as an adjective in English. The phrase the bully leader doesn't work particularly well. For day-to-day language as opposed to formal writing, the Original Poster could try a construction called a right dislocation. This is when some noun-phrase is reiterated at the end of the sentence, but with a different phrasing. This noun phrase is normally the Subject or Complement of the verb. The reiteration normally clarifies the identity of the noun phrase in some way:
- I went to see Bob Marshall, the Managing Director.
In the sentence above the phrase the Managing Director gives us a more salient description of Bob Marshall for the purposes of the story.
For the Original Poster's sentence, if they wanted to use a right dislocation they could try either of the following:
Bill helped him when he needed to overthrow the current leader, a ferocious bully.
Bill helped him when he needed to overthrow a bully, the current leader of XYZ.
I'm not sure that the adjective current works here. Current usually refers to the time of speaking. If the bully is the current leader, then it would seem that he hasn't been overthrown. The Original Poster might with to use the modifier then instead, or use the adjective previous:
Bill helped him when he needed to overthrow a bully, the previous leader of XYZ.
Bill helped him when he needed to overthrow a bully, the then leader of XYZ.
The adjective previous seem to work better, in my opinion.