"Arrow" Season 4, episode 13 saw Oliver Queen (Stephen Amell) struggle with the decision of whether to become a killer again or not. The CW

After being faced with a grim ultimatum, the heroes on the CW's "Arrow" had to decide if they're willing to bend the rules and kill one of the worst villains they've ever encountered in "Sins of the Father." However, that's not an easy question to answer once family gets involved.

Season 4, episode 13 opens where last week’s left off, with Nyssa al Ghul (Katrina Law) telling Oliver (Stephen Amell) that she’ll give his dying sister, Thea (Willa Holland), the Lotus cure. Her only condition is that he murder her father, the leader of the League of Assassins, Malcolm Merlyn (John Barrowman). Obviously, Oliver has no love for the Dark Archer since he is responsible for his best friend and ex-girlfriend’s deaths. However, the hero isn’t a killer.

Elsewhere, Felicity (Emily Bett Rickards), is having a meeting with her estranged father, Noah Kuttler (Tom Amandes). He tells her that he wants to be a part of her life and apologizes for not being around when she was a child. His excuse, being wanted by the police, is pretty lame, and Felicity doesn’t buy it. To help build her trust, he confesses to being the hacker villain the team faced last week, The Calculator. She’s understandably upset and tries to run away. Before she does, he gives her a flash drive containing verifiable records of his exploits over the years and tells her to contact him.

Meanwhile, Oliver is pitching Nyssa’s plan to Malcolm. The father insists that he would do anything to save Thea, but not if there’s no proof. He tells Oliver flat out that he won’t fall on his sword as part of “Nyssa al Ghul’s games.”

So, the hero proposes a different approach. He sends Laurel (Katie Cassidy) to talk with Nyssa and ask if she’ll settle for Merlyn simply stepping down as the leader of the League. She agrees, and offers Team Arrow proof that the Lotus cure is actually in her possession. They take a small vial of it and find that it does in fact cure Thea temporarily. The problem now is securing the whole bottle.

With proof of the cure and Thea’s condition worsening, they pitch the idea of stepping down as the head of the League to Malcolm, who agrees a little too quickly. Sure enough, when Oliver, Laurel, Diggle (David Ramsey) and Malcolm show up to surrender to Nyssa, the Dark Archer reveals that he’s double crossed everyone. Although Nyssa has a small army of loyal assassins in her command, Malcolm still commands the bulk of the League. He launches an attack and she retreats with her forces. When Malcolm tries to give chase, Team Arrow stands in his way.

"Arrow" Season 4, episode 13 pit Nyssa al Ghul (Katrina Law, left) against her arch nemesis, Malcolm Merlyn (John Barrowman, right) in a civil war between their combined forces. the CW

Not only does Merlyn’s plan not result in securing the Lotus cure for Thea, but he sparks a civil war in the middle of the civilian-filled Star City. Over the next few days, battles occur in the otherwise peaceful streets, resulting in a lot of collateral damage. Team Arrow tries desperately to get ahead of it, but keeps coming up short. Finally, Laurel tracks down Nyssa accompanied by a small group of soldiers. She tries to reason with the assassin, but Nyssa threatens to kill her instead. Just when it looks like the Black Canary has finally met her end, Malcolm’s men show up and kill everyone but Black Canary, Nyssa and her most trusted bodyguard. The three escape and a battle breaks out in the streets. The rest of the team shows up and, when the dust clears, they’ve won and have Nyssa as their captive.

Unfortunately, they can’t interrogate the answer out of her and are back to square one. Despite Diggle, Felicity, and Laurel giving him permission to kill Malcolm, Oliver still won’t. Apparently he’s holding out hope that Thea and Malcolm can end up having a good father-daughter relationship. With Thea’s condition worsening so much that she won’t survive the night, Oliver makes a final, desperate plea to Merlyn.

He tells him that he needs to challenge Nyssa to a trial by combat, to avoid the civil war and get the Lotus from her. Merlyn agrees, knowing full well that he can easily defeat Nyssa in single combat. They meet collectively on a rooftop in Star City and Merlyn and Nyssa square off. In a twist, before they can fight, Oliver reveals the true nature of his plan.

Having been forced to marry Nyssa at the end of Season 3, Oliver is still technically her husband in the eyes of the League. That means he can volunteer to fight in her place. Malcolm is a formidable opponent, but Oliver is the only man that can take him down with ease. He does so, but simply cannot make the killing blow. Merlyn says that he’ll have to kill him, because he’d rather die than relinquish the power of the League to the likes of Nyssa al Ghul. Instead, Oliver chops off his hand and knocks him out. He takes the ring from the severed appendage, which signifies control over the League, and presents it to Nyssa in exchange for the cure. This apparently fixes the entire problem without dropping a single body.

While this is happening, Felicity has been trying to figure out if she can really trust her father. She seemed to throw caution to the wind by inviting the tech thief on an all-access tour of Palmer Technologies. However, it turns out, it was all a test, which he failed miserably. Felicity took measures to stop a hacker in the building and found that The Calculator tried to steal from her. She calls him into his office later and confronts him about the deception. While she’s at it, she chews him out for making her grow up without a dad and the effect it had on her confidence. When her speech is over, she reveals that she’s had Star City PD waiting in the wings to arrest the villain. While he’s being cuffed, she leaves without looking back.

Now that the Felicity problem, the Thea problem and the civil war problem are all fixed, Oliver thinks he can relax. However, Nyssa calls a meeting with him and Merlyn to reveal that her first and last order as the leader of the League was to disband it. This sends the Dark Archer into a rage. He promises Oliver that he won’t kill him, but will make it his new mission to make his life miserable.

The episode ends with the bad guy making good in his promise. Merlyn calls a meeting with Damien Darhk (Neal McDonough) and tells the magical villain about Oliver’s estranged son, William (Jack Moore).