I disagree. We all know who the shits are. People who are abusive to their partners, will have displayed the same behaviour to most or all their previous partners because abusive people abuse. Their family and friends usually know too but they're in denial. But they know, they make excuses, that's not the same as people who know that their loved one is decent, gentle and kind.
People are who they are rapists rape, abusers abuse, liars lie... But most people aren't rapists, abusers, we judge people based on our experience of them and their reputation, that can sometimes be wrong but everybody in their life won't be wrong.
Most people are good and decent. We do know who people are and I don't trust anybody who tells me that you don't know anybody in your life. That says a lot more about them than anyone they're projecting onto.
I know what my father/brother/husband/close friends/long term exes are and aren't capable of because I know them very well. If you pay attention, you'll know, good and bad.
Why would you be friends with or marry somebody if you truly believe that he couldn't possibly be kind, loving, moral and nice because there's no such thing?
If you look at a partner and think "he could be a rapist, abuser, killer...", you're either projecting your own malevolence or you're right because you're sensing who he is and should dump him. If I felt that about someone, I'd trust my instinct and leave.
If you told me one of my parents killed someone for money, bullshit. They wouldn't do that. They have morals and character.
You project on others what you believe about yourself. If you think nobody is capable of being good and anybody is capable of doing something horrendously evil, then you're speaking of yourself and those around you, that's your truth.
There are a lot of things I wouldn't do, my family wouldn't do, my friends wouldn't do, my exes wouldn't do...
That you assume others always would, says a lot more about you and the people you're around. Which incidently, makes you very high risk for abuse because you're around people you know have no moral boundaries.
Judge people on their character and their actions. What you assume about others, without evidence, is a projection of you.