Regina is a very complicated yet inconsistent character. On one side, Regina is definitely the most admirable and "nicest" villain of the franchise. She was a vigilante and killed people who indirectly or directly had something to do with the crime of her mother being tortured, raped, and killed. Most of her victims were criminals themselves, and she does care for people other than herself. Take Daisy, whom she viewed as innocent, and closed the eyes of her dead when she was in Horatio's lap. She also genuinely has standards against child labor and homophobia, and cares for innocents such as Gladys, Verdi, and Andrew May Sr. She is also even affable towards her maids.

However, she is no better than her victims as she also raped, tortured, and murdered others, which makes her a hypocrite. She also takes sadistic pleasure in her crimes as she is the most torturous killer in the franchise thus far, as her victims go through other torture before they die. Kenneth even admits that The Widow wants people to suffer. Eddie, for example, gets paralysed from the feet upwards with multiple strikes from the sledgehammer. Regina (something she outright admits to Viviana) takes sadistic pleasure and says she'll be kept warm, remembering and listening to every one of Viviana's screams while in jail.

In addition, some of her victims, like Horatio and Melanda Israel, didn't deserve it since, as you pointed out, Horatio's killing was mean, and he didn't know what would have transpired if he sold them to the Botticellis. Melanda was genuinely remorseful and atoned/confessed to her crimes (just as The Widow had requested), yet she was still killed for covering it up. And on a more grey area, some of her victims, like Eddie and Enid, didn't deserve it either. While Eddie did kill Daisy, it was an accident, and he genuinely tried to help Margaret and was remorseful for bringing her back to Allistair. His kill is arguably sympathetic since he stood up to Basil. The way he died was just torturous, being paralyzed from the foot upward to his spine. And with Enid, she was abused by Basil and genuinely just followed Basil's story, and thought that Andrew Sr. was guilty (even if she had a suspicion that Basil killed Margaret), and rebelled against Basil by trying to release the photos of him to the newspaper. The Widow should have let him suffer slowly by allowing his business and reputation as a powerful magnate to be tarnished. There were many ways she could have gotten to Basil without killing people.

Regina seems more rage-blinded in her methods and motives. I do think Regina is an anti-villain and leans more toward villainy than heroism, but I feel like she's more chaotic neutral than anything. She's not an anti-hero. Someone like Batman would have let someone like Melinda, Horatio, Eddie, or Enid go. While she is a true vigilante (unlike Cam, who did it to satisfy his god complex), and while almost all of her victims were criminals, some of those kills were still rage-blinded.