There are many synonyms for the phrase "score to settle." One might say they have a "beef" or a "grudge" to settle with someone. Alternatively, they may have unfinished business or a matter that needs to be resolved. Some other options include having an axe to grind or a bone to pick. Another phrase that could be used is that one has a vendetta or a feud with someone. Regardless of the phrasing used, the underlying sentiment remains the same. There is an unresolved issue that needs to be addressed, and one party is determined to resolve it.