Hanlon's Razor is absolutely valid.
That said, I'd posit that stupidity isn't accidental and shouldn't be excused. Rather it should be stomped on just as hard as malice since, as others have pointed out, the effect is the same.
People have reasons for acting incompetently - often those reasons are because the system is set up to enable that, in one way or another.
And it's most often the case that system has been set up intentionally in that way (or has been manipulated to get that outcome) - there's malice in the organisational big picture.
The effect is the same and the cause is the same as well.