Regarding the OP, I don't think any of the examples there are too dark. Even the ones that you say
are too dark. If I was going to give you advice, it would mostly be along the lines of "don't resort to girls/children whenever you need a victim", because I get really tired of that - I feel like it's a cheap grab for an artificially deeper impact.
As you can probably tell by now, I don't care for horror. I don't really get "scared" by stories; my natural response to to be grossed out or turned off, but not actually terrified. But if you're going to do horror, I don't think you should hold back. Let it be what it is. Slap the NC-17 label on it and be proud (censorship standards are often bizarrely stupid anyway).
...But if there is a "too far" point, you might find that it's animal torture. Humans have a pretty profound response to that.
Well, I finally watched Imprint. While the subject matters of incest, birth defects, abortion, prostitution, child abuse and torture were unseemly, it seemed more 'in bad taste' than 'hot button issues' for me - I didn't really find the movie horrifying at all. Really it was just kind of grotesque.
<SNIP>
So I'll stay content at the levels of horror I am pursuing now as the best. Its scary and hinting at dark things, not full exposure of gross out.
Wait, is the issue about gross-out and graphical obscenity more than actual horror? Okay then, I would advise to hold back on that
heavily. As you say, it's just bad taste for the sake of bad taste, and that's more grotesque than horrifying.
Let's look at it like porn (I know, but stick with me here!). A good movie with a romantic subplot might hint heavily at sex - maybe even have a sex scene, in the typical M15+ style. Going fully into detail there would screw the film's rating, of course, but what's more important is that it would totally derail the story. As appealing as it might be to some viewers, it would ruin the pacing of the overall work and be rather gratuitously wasteful in the long run.
I think the same is true of horror. If you show that something horrible is just about to happen - or maybe show the very beginning of that horrible event before you cut to another scene - it can be better for pacing, and create more fear in the audience. Let them imagine exactly what happened - they will almost certainly understand what is implied, and chances are they can make it worse in their own mind than you can by showing it to them. The ambiguity means the true horror of the scene is limited only by the audience's imagination.
On the other hand, if you show the whole grisly affair as a play-by-play, it's just gross for the sake of being gross. It's incredibly difficult to write/direct without it seeming clumsy, tedious or fetishistic (or all three), plus it offsets the pacing and structure in the same way that the sex scene would. What I'm saying is, detailing a horror scene like that makes it what the sex scene is to the first movie - just plain gratuitous. I guess that's why they call it "gore porn" ^^;