Although, it was VERY hard lesson to learn at least Arcane realised what kind of person Stella was after she betrayed him by not believing him and naively trusting her designated SIMP "childhood friend" Edgar. Its why he makes sure the next chance he gets he will stay the hell away from her and instead protect those who stood by him in his hour of need (unlike Stella) and sacrificed everything for him. Arcane never sacrificed the lives or those who died to protect him intentionally, the reason they were killed was because dearest Stella refused to believe Aru when he tried to explain to her that Edgar was the one who framed him and was out to get him. Instead "not-like-other-girls" Stella went the extra mile with her backstabbing due to her stupidity and actually revealed the location of Arcane's hideout to Edgar who then went on to kill Arcane's friends and comrades. It's the main reason he hates Stella in this life and wants nothing to do with her or Stella to be around Dana whom he now cherishes more than anything. Keep in mind these characters are still teenagers when the events of the novel occur they aren't fully developed yet as adults. Arcane was 15 when he first met Stella and around 17 when Dana died. I agree with what you are saying regarding Dana and her enabling of Stella's toxic behavior either by making excuses for it or finding it "cute". She's not "cute" she's an arrogant, immature spoiled brat that expects everyone (especially men) to fawn over her. And gets a rude awakening when they don't. Why would anyone want to have someone like this in their life? let alone have them as a "friend"?