Hey, _OwO_!
Good to see that you're considering audience with your writing! I'll add to what owidjaja has suggested. It's really important that as a writer, you acknowledge that using sensitive subject matter bears great responsibility. If you do want to write with more touchy subjects, make sure you conduct thorough research and are illustrating it from a neutral perspective. Even something like mental health is risky because the last thing you want is to portray the topic inaccurately, whether it be by romanticising it or failing to capture its reality. I would highly recommend doing your research so you know the limitations of working with that subject matter.
A murder scene described and executed well can be used! I would perhaps justify it in your critical reflection that it is intended to reveal to your audience a deeply disturbing experience which enables them to be more sympathetic towards the victim/enraged by the perpetrator. It would just need to be treated with care so that it isn't too graphic or gorey for a marker to read. I would definitely second showing it to a trusted peer or teacher so they can confirm if it's suitable for the HSC
Hope that helps!
Toodles,
Angelina