This is one of the first writing prompts I saw on Pinterest that made me want to fix it now (which is how I got into doing these “glitch” posts in the first place), and the problem isn’t even in the grammar or punctuation. According to the basic mechanics of writing, these sentences are correct. Copyediting isn’t about just the basic mechanics of writing, though.
The second sentence of this example is too wordy, too… inflated. (It was the has the scary capability of part that always bugged me. It should just go away. If you must keep it, change capability of being to ability to be.)
“I don’t sleep. My mind can be too dark and demented.”
I’d even go so far as to recommend getting rid of and demented unless there is strong reason for the “not sane” aspect of the character to be mentioned specifically. (The word demented isn’t even being used correctly here, but since the example is dialogue… Whatever. None of my business, right? Freelance editors are so damn bossy sometimes, aren’t they, going around telling the writers who hire them how to improve their writing? *shakes head*)