Whether an action is done intentionally or unintentionally, the result will be the same. Moreover, a sin committed intentionally will have serious consequences, which are very difficult to get over.
After introducing himself to be a messenger of Sri Raama, Sri Hanuman advises and gives stern warning to Ravana, duly quoting the effects of Dharma and Adharma.
न हि धर्मविरुद्धेषु बह्वपायेषु कर्मसु।
मूलघातिषु सज्जन्ते बुद्धिमन्तो भवद्विधाः।।5.51.18।।
"Indeed, respectable people and wise men like you should not indulge in actions opposed to righteousness as it will strike at the very root of your existence.
तपस्सन्तापलब्धस्ते योऽयं धर्मपरिग्रहः।
न स नाशयितुं न्याय्य आत्मप्राणपरिग्रहः।।5.51.25।।
"You have acquired the merit of righteousness by prolonged hardships experienced during your penance. It is not proper to forfeit your own life in this way.
न तु धर्मोपसंहारमधर्मफलसंहितम्।।5.51.28।।
तदेव फलमन्वेति धर्मश्चाधर्मनाशन:।
"The fruit of dharma does not accrue to one who has reached the culmination of adharma. Unrighteousness will destroy the fruits of righteousness.
प्राप्तं धर्मफलं तावद्भवता नात्र संशयः।
फलमस्याप्यधर्मस्य क्षिप्रमेव प्रपत्स्यसे।।5.51.29।।
"There is no doubt you have earned merit through righteousness . But now you will quickly reap the result of your unrighteousness also.
So, in normal course, the act of expiation cannot nullify the sins committed. One has to reap the fruits of sins committed.
Similarly, the effects of good deeds, done with an expectation of result, cannot be undone. One has to undergo the effects of good deeds also, even if one gets vexed with them.
That is Karma Yoga. That is why Sri Krishna said to act without expecting any results.
कर्मण्येवाधिकारस्ते मा फलेषु कदाचन।
मा कर्मफलहेतुर्भूर्मा ते सङ्गोऽस्त्वकर्मणि।।2.47।।
Your right is for action alone, never for the results. Do not become the agent of the results of action. May you not have any inclination for inaction.
It does not mean that the effects of sins will always take full effect on all people. In normal circumstances, yes, but not in all cases. It is a complex subject.
If the sins were committed unintentionally and one prays to the God from one's bottom of heart, the effects may get minimised by the God, though not nullified.
The saints, the visible version of the God, can minimise or change the course of Karma, in exceptional cases. Sometimes, they may take the effect of the karma of others on to them also.
As said earlier, it is a complex subject, as to why the saints act in such a manner.
Scriptures say that one should atone for the bad deeds, a consequence of which one is undergoing bad phase of life and perform expiation.
A extract from Guru Charitra, a pArAyana text, is given below: (P.49)
As Sree Guru went on, Trivikrama asked him, "Holy Sir, if once anyone, knowingly or unknowingly, commits any of these sins, is there no means at all of salvation from the same? Is one condemned to go through this miserable round of births?"
Sree Guru smiled and replied. 'My son, Trivikrama, the sins that accrue from one's previous misdeeds can be washed off
(i) through hearty repentance,
(ii) through penitentiary religious acts and by confessing, one's sins in the public assembly and by gladly receiving the punishment thereof as per the judgement of the elders.
(iii) If one is not really capable of observing the penitentiary vows, he can save himself by giving away a cow or its price as gift to a pious brahmin.
(iv) Part of the sins can also be undone by taking the 'ten-baths' (dasa-snaana),
(v) by performing 200 Praanaayaamas and
(vi) by gifting away gold.
(vii) But by devoutly serving the Sadguru, even very great sins will vanish. Indeed, there is no better means than this.