Whatever sadly someone messed up big time.
Not necessarily. That's what the various inquiries will try and find out.
Unless of course you mean that people other than those on board might also have messed up at some point. Aircraft designers, airspace designers, Regulators, Authorities, etc, etc.
Maybe the pilots simply didn't see each other and everything else from aircraft design and blind spots to uncontrolled airspace, lack of conspicuity technology to flight authorisations, etc, etc, conspired against them.