Should I warn new/prospective PhD Student that supervisor is terrible?
You should. As you mentioned yourself, you would have loved it if someone had warned you about things before you went, and "do unto others as you would they do unto you" is a good maxim to live by.

Having said that, you should also take steps to protect yourself. Talk to the student in private, and ask for confidentiality. Do it face-to-face or via telephone/Skype if you can; if you must use email, avoid using your institutional email address. You've already acknowledged you might be biased, so tell the student the facts only and don't add anything that you inferred from the facts. For example, instead of "He does not help or even provide feedback", say how many times you meet him or attempt to contact him a week. Describe how those meetings went - what did you say, what did he say, what did you do next? Try not to say "he does not help" as though it's a bad thing: it's actually possible he is expecting you to work independently for various reasons (e.g. you were so good he doesn't think you need close supervision).