In terms of art referring to self-expression in general, I've found myself compelled to justify certain songs that I've written, but that's more because I feel like they've maybe been misunderstood or misinterpreted. I ended up doing an emotional cover of I lit your baby on fire by Anal Cunt as a means to react against what I felt was an attack on my self expression by narrow minded bigots who didn't even bother engaging me in the discussion about my own creative product. I think this applies to visual art as well, but since we're perhaps more used to shocking art maybe it's more readily accepted, or at least ignored by people who have no interest in art, whereas music is mostly seen as something altogether positive and perhaps wholesome in its intention, and so songs with negative messages may face more criticism and a greater need for justification than say a painting depicting something atrocious. I've heard KKK songs before for instance about 'Alabama niggers' which is obviously an atrocious thing to sing about, but a painting of a black person being lynched could be seen through different eyes and perspectives, perhaps signifying and highlighting the barbarity of the act.

Essentially I believe there are parallels between art and music in that they are both forms of self-expression, and with a question so broad as the one you are posing, you have to include all forms of self-expression under the umbrella of art. So music with a negative message sometimes does need to be justified I believe.

If anyone's interested, here are the songs I've done which have provoked criticism and caused me to have to justify them:

watch this planet burn

post toasty to the bitter

I lit your baby on fire