settle a score idiom (also settle old scores)
算旧账
to punish someone for something wrong that they did to you in the past and that you cannot forgive
Police believe the killer was a gang member settling a score with a rival gang. 警方认为，杀人犯是一团伙成员，当时正和敌对团伙算旧账。
He is using his wealth and power to settle old scores. 他凭借自己的财富和权力来算旧账。