The interview came shortly after ABC Entertainment president Channing Dungey announced the cancellation of Roseanne following a racist tweet Barr sent about former presidential aide Valerie Jarrett.
"She's having mental health issues right now. I'm sure that's part of this," Arnold, who also wrote for the original sitcom, told Cooper. "It doesn't make it OK. They had to cancel the show."
However, he implied that Barr wanted the show to end.
"It had to happen," Arnold told The Hollywood Reporter in regards to the show's cancellation. "And I am going to tell you the truth, she wanted it to happen, if you saw how her tweets escalated this weekend. If it hadn't happened yesterday, this season would have been so awful for everyone every day because she would have felt like she was [being] taken advantage [of], just like when I left the show."