If someone turns out not to be the way you imagined and perceived them to be, isn't that your own fault?

If they've actively worked to present a false version of themselves, then the responsibility is on them. People need to engage trust when entering a relationship, it's a natural lowering of barriers to let someone you like into your life. If they abuse that trust, then they are at fault. If you see their true nature and you decide to stay in the relationship, then that's on you.