Ratings only apply to sexual content. Violence, gore, or profanity do not affect an image's rating. Note also that ratings do not say how "family friendly", "kid safe", or "offensive" a picture may be. This is explicitly not a goal. Danbooru users are assumed to be responsible adults and the ratings are there only to help them choose the content they wish to see.
As of 2022, the above quote is now outdated. According to howto:rate, violent content is now considered alongside sexual content and the following should now be rated Explicit: