But thats the whole point - we know that he's crossed the line, but his own internal moral scale / ego / psyche is processing it differently - he still absolutely believes he is the good guy; these inhuman creatures just murdered his wife, and are getting away with it! Its the Lex syndrome on a smaller scale. He feels absolutely justified, his hate and self righteousness enabling a viscious circle where he can not psychologically admit to himself that he has crossed the line or is wrong in any way. HIS loss, HIS hate, is RIGHTEOUS - to *himself* - and he keeps proving himself right every time "they" do something terrible... like Supergirl attacking the whitehouse, or an alien murdering his wife. The deeper he goes the more desparate he gets which makes him need to try that much harder and become that much more viscious. Its not dissimilar to how i've always viewed the decent of Hal Jordan into Parallax (pre retcon). It could be fascinating but they are ignoring any depth they've set up.
Yes, he thinks he's good but he's wrong and we the audience know it. We don't need to indulge his viewpoint. I very much would like to get to Supergirl punching Nazis rather than listen to them piss and moan.
But that's just me.