SB 9.15.38

rāma rāma mahābāho
 bhavān pāpam akāraṣīt
avadhīn naradevaṁ yat
 sarva-devamayaṁ vṛthā
Synonyms: 
rāma rāma — my dear son Paraśurāma; mahābāho — O great hero; bhavān — you; pāpam — sinful activities; akāraṣīt — have executed; avadhīt — have killed; naradevam — the king; yat — who is; sarva-deva-mayam — the embodiment of all the demigods; vṛthā — unnecessarily.
Translation: 
O great hero, my dear son Paraśurāma, you have unnecessarily killed the king, who is supposed to be the embodiment of all the demigods. Thus you have committed a sin.