Sorry to disappoint you all, but "toxicity" is rarely, if ever, addressed in manga. AND on top of that...remember: THEY ARE AFFILIATED YAKUZA.
Remember that? Organized crime. Money laundering. Murder. Sexual trafficking. Drugs. Etc. They're all bad juju.
You cannot possibly be hoping for a YAKUZA heiress to find the violent tendencies of said yakuza to be "toxic" and abusive. REMEMBER WHAT SETTING THIS IS AND WHERE YOSHINO COMES FROM. She is literally in a flashback chapter telling her childhood friend she'd kill someone for him so he wouldn't have to do hard time tried as an adult. She was not bluffing, the author really wants us to understand that Yoshino doesn't bluff ya'll. Yoshino thinks she has no kidney remember. She did it TO PROVE A POINT.
If this were in any other setting, like idk...an Ao Haru Ride or Cherry Boy or something then sure. Ew, toxicity boyfriend. But we are talking about...the yakuza.
AND the only reason that yall think that the childhood friend is Best Girl is because he doesn't voice his crazy shit outloud like Kirishima. But please give the author credit for trying to telegraph through facial expression and action that homeboy ain't too different from Kirishima.
Edit: Did you guys not see the childhood friends face when he told Yoshino she could "tell him about it later".