The phrase 'おれはひとりの修羅なのだ' carries a weighty, almost mythical intensity that's challenging to capture in English. 'I am a lone warrior' might be the closest literal translation, but it loses the cultural nuance of '修羅' (shura) - a concept from Buddhist mythology representing relentless combat spirits.
Perhaps 'I walk the path of a solitary demon' better conveys the original's mix of isolation and ferocity. The word 'shura' implies someone consumed by battle lust, so adding 'cursed' or 'doomed' could deepen the meaning. Anime fans might recognize similar vibes from characters like Guts in 'Berserk' or Spike Spiegel in 'Cowboy Bebop', though neither uses this exact phrasing.