Here be massive spoilers.
Also keep in mind that I’m focusing solely on the anime.
I’ve already said that the last couple of episodes are my favorites. The main reasons for this are the aforementioned brothers and their interactions. I was hooked from the moment they showed a glimpse of Abel’s past in episode two. It took a long, long time for Cain to actually show up but he did not disappoint when he did. I don’t know what the humans who created these two were thinking, naming them Abel and Cain of all things. It’ just asking for trouble.
I have a weakness for brilliantly insane characters. I also have a fondness for complex relationships, particularly the love-to-hatred kind. In Cain and Abel, I found both to a certain degree.
Abel makes it quite apparent on multiple occasions that he wants to destroy Cain. He certainly tries his best as well. But the brief flashback in the final episode shows that at one point, they were on good terms. Cain, on the other hand, seems to still love his brother. Unfortunately, he has a very odd way of showing it. The anime doesn’t explore their past to a great extent so we’re left with only the bare minimum of details. Yes, the matter is elaborated upon in the manga and light novels but the anime is still a separate medium. We do get to see that they’re both fixated on the other, though for entirely different reasons.
Abel adopts a ‘no killing’ policy throughout the show. His revulsion for killing is so strong that he’s unable to kill Isaak despite the fact that doing so would spare thousands of lives. Yet, this restraint is cast aside the moment Cain’s survival is made apparent. Abel consistently attacks Cain with lethal intention. The anime ends with Abel setting out to hunt down Cain, stating that he’s willing to sacrifice anything to destroy him. This ditzy, peace-loving priest leaves the AX and rewrites his principles so that he can attain vengeance. While it could be said that Abel is pursuing Cain for the sake of the world rather than his grudge, I think it’s far more likely that revenge is what’s driving him. Abel has denounced vengeance in previous episodes, stating repeatedly that he believes it cannot solve anything. But that was before he knew his brother was alive and as dangerous as ever. Whenever it comes to Cain, Abel reacts emotionally and at a scale far larger than anything he has expressed previously. Principles and restraint all fall away at that point.
And then we have Cain. Where do I even start with Cain? He doesn’t get much screen time and as in Abel’s case, only a little of his past is shown. But from what we do see, it appears that Cain used to be a relatively decent person. We also know that sometime after he became a Crusnik, he killed the fourth Crusnik, Lilith, believing it to be a favor for Abel. This act is why Abel wants vengeance. While Abel exhibits straightforward rage and hatred towards his brother, Cain’s feelings seem a little more complex. In fact, he seems to have a singular obsession with his twin. He follows a declaration of affection with a killing blow. Later, he tries to fuse their bodies into one so that they can always be together and Cain can gain a fully functional body. You could say that Cain’s general insanity is most clearly evident in his ‘love’ for his brother.
To me, the broken and twisted bond between these brothers is the highlight of the series. The whole ‘revenge for a murdered love’ thing has been done to death but in this case, the shared history of the Crusniks makes it interesting nonetheless. I have hunted down spoilers for the light novels and manga, especially those concerned with Abel’s past, and what I have found only serves to make the entire affair more tragic. I’m disappointed that the anime didn’t explore it further but I guess they can’t squeeze everything into a single season.