I'll give it around 7.5.
It has elements of a great story but the lack of connection between some of the last scenes bumped the score down. So story: 7.
I can't adequately judge the score of the ONA but I enjoyed it. Sound: 8
Understandably, with such a short clip, you cannot fleshed the characters out really well. Character: 6.
Overall, in my opinion, it's very short but quite enjoyable. Albeit not a gore-fest or a violent movie, it's not everyone's taste. Hell, it's not to MY tastes even. But I enjoyed it, which made it a good viewing experience.