The reason why we judge people by their action and judge ourselves by our intentions is that we oftentimes are ignorant of the intention of people that makes them take certain actions, so sometimes the intention may be good but the action which is needed to bring about the actualization of the intention may be wrong and at the end of the day we only see the actions which is gone wrong and we then even neglect to ask people what the purpose or intent of their action was. But in our cases, we often already know the intent of our actions and so if we fail in trying to accomplish something through questionable actions we would justify ourselves with statement like "it's for the greater good" of which we term the intention.
So we as humans are quick to justify our actions because we feel our intentions maybe wrong even when we apply the wrong or unethical methodology but when it's someone's else, we hardly do these because people's wrong are always the focus and the reason why they were indulgent in the wrongdoing sometimes isn't always considered when placing or laying blames.
So we're obviously flawed, sometimes we can be naturally biased but unknowingly too, so we're often quick to conclude when it comes to others but we're often quick to find reasons when we are the ones that have gone wrong. I hope I've answered your question?
The selfishness and wickedness of man makes man judge others by their actions and self by their intentions!! It could also be as a result of Ignorance of self and others!!
Selfishness, makes them to see evil in others actions while beholding only good in theirs intentions and actions. Thus , all efforts should be made to consciously see "good" in every activities of others , while striving to God nvolve self in activities which bulids rather than destroy others!!