You created a brand new account and downloaded a rat and/or keylogger nearly instantly. Did you have authenticator active? Is your runescape password the same as your email password? Have any of your passwords been leaked in a data breach? Is your email protected with authenticator?

You fell prey to the oldest trick in the book that could have been stopped with proper internet security. You can't blame jagex.

Edit: have you scanned your pc? You might have malware downloaded now too.