Robert J. Hanlon had a razor of logic.
It was a joke.
At least, he is said to have submitted it to a joke book.
(Had to move from the Wikipedia page to his Wikiquote page to find out what joke book. He is (apparently?) quoted in one of Arthur Bloch's Murphy's Law books. Not enough information. Bit of irony in that.)
Jokes usually have a root in reality. That's what makes them funny -- they offer an unusual and amusing viewpoint of some painful point we usually take for granted.
Anger is something we take for granted. Assignment of blame is something we take for granted. Defense and revenge are also things we take way too much for granted.
The joke goes like this:
Never attribute to malice that which is adequately explained by stupidity.
I've also heard it said as incompetence rather than stupidity.
Either way, stupidity and incompetence are not particularly more friendly charges than malice. Different, but not a whole lot better to be saying about others, especially without solid proof.
But I note that Hanlon is not quoted as saying whose stupidity. The quote does allow being read as attributing things we don't like to mutual stupidity.
It's never good to make up rules to call yourself stupid. If you can think well enough to call yourself stupid, you are trying, even if others think you are not. And trying is the first thing that moves you beyond stupidity.
So I offer my own razor:
Better to assume both you and the other guy are less than perfect than to assume he or she is out to get you.
And I'll offer a twin to that:
If you demand perfection of the other guy, don't be surprised when he or she demands it of you.
God is an apparent exception, but our own ideas of perfection are enough removed from real perfection that it is going to be more useful to forgive the Gods for being stupid rather than decide that they are out to get you. (Or, if you don't believe you believe in God, forgive the universe for being stupid rather than assume it is has a grudge against you.)
While I'm at it, I'll offer a close cousin that is very appropriate to our current political climate:
If the other side is getting excited, they probably think they have a reason. If they think they have a reason, we really should consider the possibility that they do, and that the reason lies, in part, in us.
Anyway, in spite of the fact that real abuse and offenses occur, we should be looking for reasons to forgive instead of reasons for war -- again, noting that forgiveness and permission are separate things.
You can forgive an abuser and still get yourself away from him or her.
In extreme cases, you can still seek an injunction or court order stopping somebody from doing something that harms you, even while you forgive the person for being unable (for malice, stupidity, or whatever reason) to refrain from harming you if he or she gets too close to you.
We're all imperfect. Stupidity is less a disease that is catching than it is a state of being that we all share in.