Here is what the Manusmriti 11.41 says:
If a Brāhmaṇa who has set up the fires neglects them wilfully, he shall perform the ‘Cāndrāyaṇa’ penance for a month; as his offence is equal to the offence of killing a hero.—(41)
Here is the verse in Sanskrit:
agnihotryapavidhyāgnīn brāhmaṇaḥ kāmakārataḥ | cāndrāyaṇaṃ caren māsaṃ vīrahatyāsamaṃ hi tat
Now, vira means 'hero', and hatya means 'killing', so virahatya means 'killing of a hero'.
But my question is, what does 'hero' mean in the context of this sin?