system
Given the following post, respond as if you're commenting on Reddit:
human
[jojo] How does killing DIO save Holly?

DIO gives himself a stand and because he has Jonathan’s body, the members of the Joestar Bloodline manifest stands too. Holly isn’t strong enough to handle a stand so it’s slowly killing her. How does killing DIO save her? If his death will weaken her stand, why doesn’t it weaken Star Platinum, Hermit Purple, or Crazy Diamond (who was also killing a young Josuke before DIO was killed)?
gpt
Joseph Joestar explains this near the beginning. When Dio was inert and not fed they were fine. He's recovering, trying to regain/get more power. And that was having a side-effect of manifesting stands in Joestars, all of them we know of at the time which included Jotaro, Joseph, and Holly. DIO himself mentioned this was an unintended and unfortunate side effect since he didn't want anyone fucking with him while he recovered/gained power. Joseph presumed if they put him down, that the stand that Holly hadn't tried to harness and control would calm down since it was only manifesting in response to DIO. 

He was right.