SB 4.11.7

manur uvāca
alaṁ vatsātiroṣeṇa
 tamo-dvāreṇa pāpmanā
yena puṇya-janān etān
 avadhīs tvam anāgasaḥ
Word for word: 
manuḥ uvāca — Manu said; alam — enough; vatsa — my dear boy; atiroṣeṇa — with excessive anger; tamaḥ-dvāreṇa — the path of ignorance; pāpmanā — sinful; yena — by which; puṇya-janān — the Yakṣas; etān — all these; avadhīḥ — you have killed; tvam — you; anāgasaḥ — offenseless.
Translation: 
Lord Manu said: My dear son, please stop. It is not good to become unnecessarily angry — it is the path to hellish life. Now you are going beyond the limit by killing Yakṣas who are actually not offenders.
Purport: 

In this verse the word atiroṣeṇa means “with unnecessary anger.” When Dhruva Mahārāja went beyond the limits of necessary anger, his grandfather, Svāyambhuva Manu, immediately came to protect him from further sinful action. From this we can understand that killing is not bad, but when killing is done unnecessarily or when an offenseless person is killed, such killing opens the path to hell. Dhruva Mahārāja was saved from such sinful action because he was a great devotee.

A kṣatriya is allowed to kill only for maintenance of the law and order of the state; he is not allowed to kill or commit violence without reason. Violence is certainly a path leading to a hellish condition of life, but it is also required for maintenance of the law and order of the state. Here Lord Manu prohibited Dhruva Mahārāja from killing the Yakṣas because only one of them was punishable for killing his brother, Uttama; not all of the Yakṣa citizens were punishable. We find in modern warfare, however, that attacks are made upon innocent citizens who are without fault. According to the law of Manu, such warfare is a most sinful activity. Furthermore, at the present moment civilized nations are unnecessarily maintaining many slaughterhouses for killing innocent animals. When a nation is attacked by its enemies, the wholesale slaughter of the citizens should be taken as a reaction to their own sinful activities. That is nature’s law.