A large percentage of murders are committed by people with prior criminal records
https://jamanetwork.com/journals/jama/fullarticle/201308
Results For 1990-2000, 42.6% of 884 cases had at least 1 felony conviction compared with 3.9% of nearly 7.9 million controls, for a population-attributable risk of 40.3% (95% CI, 37.0%-43.8%); among cases, 71.6% had experienced any arrest from 1990-2000 compared with 18.2% of controls, for a population-attributable risk of 65.3% (95% CI, 61.6%-68.8%).
In the big cities - I’ve looked at the Police Annual reports for Chicago, New York and LA - the percentage of murders by people with a prior record is 80%, and the VICTIMS usually slightly HIGHER record of arrests: 80-85%.