- "They're not people, they're criminals."
- —Isaac attempting to justify his actions[src]
At some point, Isaac became the apprentice and partner of Ted Grant, the two were vigilantes operating within the Glades of Starling City. However, one day, Isaac beat a drug dealer to death with brass knuckles. Ted subsequently cut off all ties with Isaac, telling him to leave and never return to Starling City.
Vengeance on Ted Grant
Isaac began killing criminals and strung up their bodies in Ted's gym and vigilante hideout. Attracting the attention of Oliver Queen, Oliver and Ted began looking for him. Isaac managed to escape and kidnapped both Laurel Lance and Ted and forced them in a car. Laurel managed to call Team Arrow who came to their aid. In the ensuing confrontation he fell out of the car and faced Roy Harper. The two had a hard fight after Isaac got him in a chokehold and told him Oliver just considers him like another weapon in his arsenal, and would give up on him eventually. Roy escaped his hold and managed to knock him out, after which he told him he was not him; in other words, Ted fired him because he brought it on himself.
Isaac was later seen being escorted by two police officers. Carrie Cutter showed up and took out the two cops. Identifying herself as "Cupid", Carrie shot an arrow at Isaac, killing him. Carrie then dressed Issac's body up with a costume similar to that of the Arrow. Issac was later found by Quentin Lance and the Arrow.
- "Don’t you get it? He’s using you! You’re not a human being to him, you are just another weapon in his arsenal and the second you do something wrong, he’ll turn his back on you. He will abandon you."
"I’m not you."
- —Isaac Stanzler and Roy Harper[src]
Initially, Isaac was impulsive and reckless, yet had a sense of justice and honor, and was once a good-hearted individual, until he crossed the line, when he beat a drug dealer to death, this action of his was what led to him being tortured by the Culebra gang for months as revenge for the death of the drug dealer that he killed.
After this, Isaac became angry, vengeful, ruthless, unstable and a hypocrite, instead of facing the consequences of his actions, he tried to blame Ted Grant, his former mentor and partner, for his actions, he was also not above killing innocent individuals in his quest for revenge against Ted, he also tried to get into an individual's head, like when he stated to Roy Harper (under the persona of Arsenal) that the Arrow "just considers you like another weapon in his arsenal, and would give up on you eventually"; which ultimately backfired.
- Expert hand-to-hand combatant: Much like his former mentor, Isaac was highly skilled in boxing and hand-to-hand combat; being able to kill multiple armed gangsters by beating them to death with brass knuckles. He was able to almost fight on par against Roy Harper (under the persona of Arsenal) but was eventually overpowered by him.
- Skilled marksman: Isaac possessed some skills in the use of firearms.
- Stealth: As a former vigilante, Isaac had the ability to enter and leave an area without being noticed. He was able to infiltrate an area, kill multiple mobsters, capture one and leave without getting detected by Team Arrow. He was also able to sneak up on his former mentor, Ted Grant.
- Guns: Isaac was very proficient in using guns.
- Brass Knuckles: Isaac often used a pair of brass knuckles against his opponents.
Behind the scenes
- Isaac Stanzler is similar to Vengeance, a minor DC villain who is murderous vigilante and an enemy of Green Arrow. Clearly inspired by antics of the Vigilante, Adrian Chase, an otherwise unnamed man used a similar costume (which is also similar to Grant's and Stanzler's vigilante garb in the series) and a gun, had a specific message to each of the victims ("Vengeance", similar to Stanzler's "Guilty") and was also murdered by Carrie Cutter, Cupid.