I talk to them, I don't know how to fix it and I would if I could but I try to be there for them. If only so someone will listen.
You don't get it, do you? People aren't careful when they cut, they don't care about themselves that much. I know people who only cut because they can't get up the nerve to kill themselves. So yeah, they are destroying themselves, they could very easily cut too deep and not realize and start bleeding out then not know what to do. Or be too ashamed or embarrassed to get help. How is endangering your life not destructive?
I'm sorry people don't try to help you, but do you ever think they are frightened and don't know how to handle it? Or perhaps they are trying to protect themselves because they aren't strong enough. Unfortunately, people aren't always able to make the good choice, and instead they take the choice best for them. And it'd be nice it they were able to step up, but they don't have to and they could burn themselves out taking care of everyone else. And that's terrible too.
And I personally think people who are cutting or doing anything else destructive to deal with pain need to go to therapy for their problems so they can talk through them and find a new solution other than harming yourself. You don't need to be drugged up in most cases, but you need to talk to a professional that can help you understand how you're feeling and give you mental tools to work through your problems.
And I wish you had a friend who was both willing and capable of helping you, but even if you don't, that doesn't mean you can't find a better way to deal with them. And not everyone who is anorexic needs to be told they're pretty, it's not always about looks as it is about control. You can choose if and when you eat, and to prove to herself that she has some control she chooses not to eat. It's not that in every case, but in some it is and I don't know her exact problem, but I'm just saying an alternative to her doing it to feel pretty. As for your friend throwing himself off trees, that isn't safe, is he doing it for thrill seeking? I don't really understand a whole lot about stuff like this, but both of those are dangerous, and they need to find other ways to face their problems, to make themselves feel in control or pretty or alive or whatever they're trying to feel through the activities.
And as for why they aren't trying to be supportive of you in return, I can't honestly say, but my best guess is they may be relying on you for strength and it scares them when you are going down a destructive path similar to theirs. And that's not fair to you, but that's just my guess.
When I tell someone they need to go to therapy, I'm not saying it in a mean way like I don't care, I'm saying it because I do care, I don't want to see them falling apart and I know I'm not capable of fixing their problems, but maybe a professional could help them find the tools in themselves to deal with their problems. You know what I'm trying to say?