From Longman Dictionary of Contemporary Englishpay/settle an old scorepay/settle an old scorePUNISHto punish someone for something wrong that they did to you in the past → old
Examples from the Corpus
pay/settle an old score• Oh, I heard plenty of rumours, but they were nearly all based on settling old scores.• There was no place like the thick of battle for settling an old score.• With the championship having been decided, this was likely to be their last chance to settle old scores. 