Well, the thing is, there's not
really a 'dark route.' There's dark decisions, and those decisions have individual consequences. You can be the nicest guy in the world to everyone, except one girl. And how deeply that girl impacts your MC's life will vary based on their relationship.
If you chose Viola's dark scene in her first appearance, she's basically gone. We'll hear about her a bit, but this pair of images kind of shows how that looks in practice. In one version, you see Viola on a bench after she's just finished a shift at work. So, you can choose to stop and talk to her, or drive off and leave her. In another version, you never see her, because she never returned to work after that night.
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Now, what effect does this really have on MC? Not a big one. To him, she was just some nobody he ran across in the course of his day. If he's on Gabby's good path, but Viola's dark path, you may feel some reverberations. And there'll be some scenes that will play out differently with another character. But, they'll be relatively minimal.
It's like the multiverse. You have a single timeline, and then you make decisions, and that timeline branches. Sometimes you make a little decision and it branches and rejoins the main timeline, and sometimes you make a huge decision and big changes happen, putting you in an alternate universe. Big acts of good or evil are likely to have big effects, though.
I don't want to give too much away, and I don't want to discuss it too much. People will want to jump to conclusions, and I'd rather they just wait and play the updates.