Under what conditions can a "good" or "charitable" act actually be sin?

I think John 18:10 best exemplifies "doing good while sinning."

Then Simon Peter, who had a sword, drew it and struck the high priest's servant, cutting off his right ear. (The servant's name was Malchus.)

Here, Simon Peter used physical violence to prevent a mob from taking Jesus to Pontius Pilate.

Who among us could blame Simon Peter for doing this? Yet Jesus explicitly instructed his disciples to let the prophecy be fulfilled. He urged restraint. And yet, with the best of intentions, Simon Peter disobeyed Jesus, and did good while sinning.

Any good or charitable act done for self-serving purposes is rejected by God and therefore could be considered sinful (though in this case it is not the act that is sinful so much as it is the person who is performing the act is sinning in the process of committing it). Jesus said of these people "The have their reward".

Also, a good or well intended act forbidden by God such as Uzzah who sinned by the "good" work of trying to stop the Ark from touching the ground.

Finally, the argument could be made a good or well intended act that is outside the will of God for you or for that situation such as Moses striking the rock at Horeb. Providing water for the people was good, but outside of God's will Who wanted the Isrealites to trust Him.

Granted, it could also be argued that any act that is sinful cannot be considered "good" no matter the intent behind it, so there is that, too.

While I'd never be capable of judging this, we Orthodox do pray to be forgiven of both intentional and unintentional sins. This would imply that it is not only the intent of the act which is important but also the result. 'Sin' in our case means failing to be what we are intended to be, to literally 'miss the mark'.

The Catholic tradition of moral theology addresses this question in significant detail. The following is an extremely rough summary, but generally speaking one can distinguish between three aspects of a moral action:

• Object: the thing you're actually doing (e.g., giving a coin to a beggar)
• Intention: the end toward which an action is aimed. Note that a single action can be, and often is, inspired by multiple intentions (the relief of the beggar's hunger, but also in order to be numbered among the sheep)
• Circumstances: I'm having trouble giving a simple definition but these are "accidental" characteristics of an action which nevertheless contribute to its moral character. St. Thomas Aquinas calls them "proper accidents", a term which he probably learned from Aristotle. The modern Catechism mentions the consequences of an action as among the circumstances. Another morally noteworthy circumstance might be compulsion -- either interior like addiction, or exterior like a threat of force.

You can find all of this articulated in St. Thomas Aquinas (particularly the question in the Summa on "The good and evil of human acts, in general") or, more recently, in the Catechism's article entitled "The Morality of Human Acts".

At any rate it's been the typical understanding among Catholics for a long time that for a moral action to be truly good, all three of the above must be good.

Conversely, for example, a sinful intention (so that I can boast) can make an otherwise good action (giving coins to a beggar) into a sin.

That leads to the second part of your question:

How would these teachings define sin in such a way that even an act of deliberate charity could be considered sinful?

The Catechism has this on the definition of sin:

Sin is ... failure in genuine love for God and neighbor caused by a perverse attachment to certain goods

So I guess the application to your example might be something like this: The man who gives a coin to a beggar out of pride is perversely attached to his own self-image as a righteous man and this attachment prevents him from acting in a way that is truly loving either toward the beggar or toward God. One could fairly easily imagine such a man walking right past the beggar on his way toward a more promising opportunity to justify himself.

And the Catholic ethicist would say that if the man is acting from pride he is not performing an act of charity (even though, concretely speaking, the beggar still gets a coin which is an objective good that results from the action).

Finally, one last note:

attitude towards sin that excuses misbehavior if a person had "good intentions", most Christian traditions do not seem to include the persons feelings in the matter when defining an action as sin or not

It's worth nothing that in a strict analysis according to the three traditional Catholic ethical principles above, the actor's feelings -- in the sense of the subjective state of his bodily passions -- would be part of the circumstances, if they were to be considered at all. The intention in Catholic moral thought is the end toward which the action is ordered, and has nothing at all to do with feelings.

The Catholic position holds that for grave (mortal) sins, one must be fully aware of the sinful act and commit it with full intention of doing so. If you have not, then you aren't culpable. Venial (minor) sins, looking at the catechism, doesn't appear to be as clearly specified so I think it's reasonable to assume that these may be committed inadvertently. However, while social ramifications may occur despite your intentions and knowledge, they aren't necessarily incredibly serious offenses either. As mentioned in the Catechism:

1873 The root of all sins lies in man's heart. The kinds and the gravity of sins are determined principally by their objects.

So, if one commits an action that would be sinful, but has not sinned in one's heart (if it was unintentional or accidental through no fault of your own) then you are not culpable for the sin. Conversely, if one performs a good deed yet does so for reasons that are amoral, the person would have sinned.

An example of this would be Jesus' rebuke of the Pharisees. They acted according to the letter of the Law because they were commanded to uphold it. However, in their hearts, they harbored pride and greed, among other sinful thoughts. In Matthew 23:27-28, Jesus describes this with a metaphor:

27 Woe to you, scribes and Pharisees, hypocrites; because you are like to whited sepulchres, which outwardly appear to men beautiful but within are full of dead men's bones and of all filthiness. 28 So you also outwardly indeed appear to men just: but inwardly you are full of hypocrisy and iniquity.

Jesus also states earlier in this chapter, in verse 23:

23 Woe to you, scribes and Pharisees, hypocrites; because you tithe mint and anise and cumin and have left the weightier things of the law: judgment and mercy and faith. These things you ought to have done and not to leave those undone.

Here this shows that they have sinned in their attitudes, but even so, their works were good. So what Jesus is calling them to do is to reform their hearts, but continue to do good works as they have been.