Technically Cain has alway show his lust toward Aria, both the first timeline and this second timeline. Aria has said it herself, that due to the time she fooling around with man in her first timeline, she recognise how man gaze upon her in lust. Only that in the first timeline, she just alway think of Cain as her brother so she'd never considered it that way. Cain in first timeline angry with Aria fooling around with man, jealous at Aria getting close to others but himself cant get any of that, at the end turn to rage and want to kill Aria.
Now Cain in this second timeline is not any better, still give lustful gaze to Aria, but the different is Aria instead realize what he has inside his gaze, fake a friendly attitude to Cain. So with another option, Cain just happily indulge himself with his obsession and keep wanting more, instead of get angry for not getting Aria, he now get angry for letting Aria away from his hand.
On plus side, Cain are just basic noble-selfish, coward, corrupted. He still stay afraid of higher noble power, still rely on underhand trick, but he do realise what he trying to do is bad - but still do it anyway. He just had a bad father, a bad personality himself, and the worst possible sister - even 2 of them.