If a law punished a specific murderer or group of murderers it would be, but not if it punished an entire defined class of murderers equally. Lawmakers generally describe the qualifications of a crime (e.g. “anyone who knowingly and willfully causes the death of another person") and prescribe a punishment for anyone convicted of it, and then the courts determine whether or not individuals are guilty by whether they meet the qualifications laid out in the law. Bills of attainder are laws that define individuals or specific groups as guilty by identity, not qualification, which means they don’t get a trial and the judiciary doesn’t get to act as a check on the legislature.
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If a law punished a specific murderer, it would be. Lawmakers generally describe the qualifications of a crime (e.g. “anyone who knowingly and willfully causes the death of another person") and prescribe a punishment for anyone convicted of it, and then the courts determine whether or not individuals are guilty by whether they meet the qualifications laid out in the law. Bills of attainder are laws that define individuals or specific groups as guilty by identity, not qualification, which means they don’t get a trial and the judiciary doesn’t get to act as a check on the legislature.