Have a friend with issues who got back into it when she hit a really rough patch. Whipped out a razor in front of me one day and started going at her arm to "freshen" the scar (she wasn't doing herself much harm, but she was drawing blood, for sure). I really felt like she might be trying to get a reaction, which, fuck that shit; you're a grown-up, use your damn words. And I was doing plenty for her already. Anyway, I just kinda looked at her and dryly asked when the last time we had boundaries was.
Scarification is an even finer line than tattoos between art and NSSI. Which is a whole nother topic or two I could rant about for awhile but won't.