physical pain makes emotional/mental pain feel dull, of course, it does seem that they were just seeking attention, since you seem to have found out about them doing it...
depends with the person tbh, but don't say that everyone that cuts is doing it for attention, because that's just a passing on regurgitated bullshit, some people with major depression/painful memories are really just trying to stop the horrible mental ache
oh, and don't feel guilty, you're not responsible for other people's self esteem, if they honestly weren't seeking attention, they didn't cut because you broke things up, they cut because they need to work out issues with themselves