Yet Ranma didn't respond. She had fallen asleep in Akanes
arms and was snoring away contentedly. Akane smiled, laid
Ranma down on her futon and covered her with a blanket
before slowly leaving the room.

It would be effective later to have had it occur to Akane here
that she should stay and not leave Ranma alone.

When Ranma opened her eyes, a lovely scene greeted her. She
was no longer in her room or the Tendo's residence for that
matter. She was laying on the top of a grassy hill covered
in flowers with a very large and full moon hanging just off
the horizon. The stars were bright and twinkling, blanketing
the night sky.

Never in her life had Ranma been to such a place. Even in
all her travels, nowhere she had been even compared to it.
She stood up and began to take in her surroundings. The
grassy hill stretched as far as she could see. But Ranma
didn't complain. She headed down the slope toward the moon
and thought of what had been happening. Everything was a
blur.

I found this section effective, though I'm not sure why.  I keep
thinking of "improvements" I would make to it, then wonder if
they wouldn't actually make this worse.  Possibly make it a
bit more explicit that Ranma associates this image with the
afterlife?  Try to make us feel WHY Ranma feels this is the
most lovely scene she's ever seen?

Two callused and clawed hands reached out of the shadows
toward Ranma. She spun around and tried to run but fell on
her face. She looked down at her self in alarm. She was
bound, hand and foot, and was completely nude. Her head shot
up and she looked at the advancing figure, shaking her head.
"Please...not again..." she begged, the tears flowing. "Please..."
she tried violently to loosen her bonds but to no avail.

She felt a prick on the side of her neck and a hypodermic
needle fell to the ground. Her vision blurred and she shut
her eyes as the hands took shape and finally reached her.

This dream didn't quite work for me.  Other than the needle,
it re-opens the question of whether Ranma was actually
physically raped, BECAUSE it's too dream-like.

She crossed her arms over her knees and began to think. She
thought about what happened...whether or not she could live
with it, what this would do to her manhood, what her mother
would think.

This paragraph would be a lot more effective and have a lot 
more impact if
you went into detail.  Instead of telling us that she thought 
about these
things... show us what she's thinking.  Remembering the man 
on top of her,
denying that it could have happened... feeling the revulsion, 
the filth... I
would think Ranma's reaction would be even more violent than a normal
woman's.  Does he blame it on his female self?  Is he 
suddenly terrified of
other men, of being touched by other men?  Maybe even in some 
way sickened
at his own male self?  Maybe wanting immediate revenge?  Give 
us a bit more
than "he thought about it."  ^_^

Miko is dead right here.  We have to feel why Ranma would conclude
that the rest of his life is not worth living, or that he
is somehow obligated to commit suicide immediately, or both.

Sorry for the abrupt ending but I wanted to get this done.
It is my first dark fic so it is not very good. Sorry again.
Hope you enjoyed.

I wouldn't say that it's 'not very good'.  You write well... 
but that is an
amazingly abrupt ending, one you chose merely to get out of 
having to write
anything more.  You know that makes it an unsatisfying ending 
as well...
Even if you don't write a different ending, you owe it to 
yourself and your
readers to at least put as much work into the end of the 
story as you put
into the beginning of it... and the story won't be complete 
without a little
more information about HOW Ranma was raped, and, perhaps, WHO 
as well.  In
short, I think you write well but you're shortchanging your 
own story by not
developing it enough and not allowing the plot to fully develop.

Again, I agree with Miko.  You DO write well, but, to be frank,
you were lazy in this story.  The body of the story doesn't
justify the ending, and consequently, your readers don't feel
what they should when they get to the ending.  In other words,
the story is not really "dark" at all, because your readers
don't believe the ending.

You don't really need to tell us who raped Ranma, and how, if
you have some reason not to, but you DO have to convince us
that it happened (and explaining how is the easiest way to do
that).

More to the point, I think you need to have more time elapse
between rape and suicide.  Have a few more unfortunate events
happen to Ranma, such as actually meeting his mother.

You have some of the materials for a very good, effective
story here.  Just put the rest of the work it needs into it,
and you might end up with a masterpiece.

If you find it difficult to put this amount of work into a
darkfic (I would), there is an alternative.  Many fanfics find
it easy to play on our sympathy for onna-Ranma simply by
having her sitting and crying.  This works for me while I'm
reading it, but often not after I've had a bit of time to
think about it.

What would be MORE effective, I think, but I almost never
see done, is to play female Ranma as a "male" personality:
have her grit her teeth, spit out the mud, wipe away the
blood, and fight back.  In the context of your story, what
I'm suggesting is that you change the ending completely
(it's not clear that you like it yourself).  End in revenge
against the rapist.  Have Ranma reconsider her gender
identity because of the rape, consider having Nodoka and
Genma reject Ranma, but KEEP RANMA FIGHTING BACK.