Both Katie and this Ellie chick were doing drugs. Ellie was introduced to drugs by the producer of the show who also groomed her when she was underage. Ellie's sister covered the whole thing up. TLDR: I think drugs often remove the inihibition controls which prevent you from killing yourself when you want to (most normal people when cutting themselves or whatever, get a strong fear "I want to live" response). But people under the influence of drugs can easily hang themselves or cut their wrists whilst in a drugged state and not notice what's happening and/or the pain. I'm pretty sure that's how Katie ended herself (drugs then whatever she did to off herself). Pretty obvious in her last X comments that she wasn't even fully aware of her suicide actions and even lamented "whelp I guess it's really too late now/etc".