This is a crime that only sinful transgressor dares to do, so he deserves both characteristics together:

مُعْتَدٍ (sinful): Who defies the truth, overtakes the limit by his assault on Allah’s (swt) religion and legislation.

أَثِيمٍ (transgressor): In Arabic it is an excessive form of sin. Allah (swt) did not say آثم to indicate that he dared to sin and persisted in it and became used to it until the sin became a characteristic to him.