Punishing the evildoers is a prime duty of a kshatriya (the ruler of the nation)

नाधर्मो विद्यते कश्चित् शत्रुन्हत्वातत्तायिन:
अधर्म्यंयशस्यं च शात्रवाणाम प्रयाचानम — महाभारत ५.३.२१
No sin arises out of killing a terrorizing enemy; on the contrary pleading before it is unrighteous and an insult to one’s reputation.