I won't accept my own answer.
I've found the following suttas (MN 61 and MN 88) that indirectly answer the question.
These suttas state that unskillful deeds by body, speech and mind, hurts myself, hurts others or hurts both, is blameworthy (i.e. invites criticism from others), and has suffering as its outcome and its result.
Although these suttas do not talk about regret or remorse, I would say, one would regret performing actions by body, speech and mind that brings suffering to oneself and/or others. It goes back to the first noble truth, that there is suffering. And we don't want suffering.
So, using this definition, we can derive virtues and the opposite of virtues.
This commentary by Ven. Sujato for MN 88 is very apt:
“Unskillful behavior.” “Unskillful” (akusala) is an unwholesome
quality of the mind, so this is the psychological dimension of
immorality. What is unskillful is that which leads to the result
opposite to that desired: we want happiness, but it leads to
suffering.
Quotes from MN 61, followed by MN 88:
“In the same way, deeds of body, speech, and mind should be done only
after repeated checking.
When you want to act with the body, you should check on that same
deed: ‘Does this act with the body that I want to do lead to hurting
myself, hurting others, or hurting both? Is it unskillful, with
suffering as its outcome and result?’ If, while checking in this way,
you know: ‘This act with the body that I want to do leads to hurting
myself, hurting others, or hurting both. It’s unskillful, with
suffering as its outcome and result.’ To the best of your ability,
Rāhula, you should not do such a deed. But if, while checking in this
way, you know: ‘This act with the body that I want to do doesn’t lead
to hurting myself, hurting others, or hurting both. It’s skillful,
with happiness as its outcome and result.’ Then, Rāhula, you should do
such a deed.
While you are acting with the body, you should check on that same act:
‘Does this act with the body that I am doing lead to hurting myself,
hurting others, or hurting both? Is it unskillful, with suffering as
its outcome and result?’ If, while checking in this way, you know:
‘This act with the body that I am doing leads to hurting myself,
hurting others, or hurting both. It’s unskillful, with suffering as
its outcome and result.’ Then, Rāhula, you should desist from such a
deed. But if, while checking in this way, you know: ‘This act with the
body that I am doing doesn’t lead to hurting myself, hurting others,
or hurting both. It’s skillful, with happiness as its outcome and
result.’ Then, Rāhula, you should continue doing such a deed.
After you have acted with the body, you should check on that same act:
‘Does this act with the body that I have done lead to hurting myself,
hurting others, or hurting both? Is it unskillful, with suffering as
its outcome and result?’ If, while checking in this way, you know:
‘This act with the body that I have done leads to hurting myself,
hurting others, or hurting both. It’s unskillful, with suffering as
its outcome and result.’ Then, Rāhula, you should confess, reveal, and
clarify such a deed to the Teacher or a sensible spiritual companion.
And having revealed it you should restrain yourself in future. But if,
while checking in this way, you know: ‘This act with the body that I
have done doesn’t lead to hurting myself, hurting others, or hurting
both. It’s skillful, with happiness as its outcome and result.’ Then,
Rāhula, you should live in rapture and joy because of this, training
day and night in skillful qualities.
When you want to act with speech, you should check on that same deed:
‘Does this act of speech that I want to do lead to hurting myself,
hurting others, or hurting both? …’ …
If, while checking in this way, you know: ‘This act of speech that I
have done leads to hurting myself, hurting others, or hurting both.
It’s unskillful, with suffering as its outcome and result.’ Then,
Rāhula, you should confess, reveal, and clarify such a deed to the
Teacher or a sensible spiritual companion. And having revealed it you
should restrain yourself in future. But if, while checking in this
way, you know: ‘This act of speech that I have done doesn’t lead to
hurting myself, hurting others, or hurting both. It’s skillful, with
happiness as its outcome and result.’ Then, Rāhula, you should live in
rapture and joy because of this, training day and night in skillful
qualities.
When you want to act with the mind, you should check on that same
deed: ‘Does this act of mind that I want to do lead to hurting myself,
hurting others, or hurting both? …’ …
If, while checking in this way, you know: ‘This act of mind that I
have done leads to hurting myself, hurting others, or hurting both.
It’s unskillful, with suffering as its outcome and result.’ Then,
Rāhula, you should be horrified, repelled, and disgusted by that deed.
And being repelled, you should restrain yourself in future. But if,
while checking in this way, you know: ‘This act with the mind that I
have done doesn’t lead to hurting myself, hurting others, or hurting
both. It’s skillful, with happiness as its outcome and result.’ Then,
Rāhula, you should live in rapture and joy because of this, training
day and night in skillful qualities.
All the ascetics and brahmins of the past, future, and present who
purify their physical, verbal, and mental actions do so after
repeatedly checking. So Rāhula, you should train yourself like this:
‘I will purify my physical, verbal, and mental actions after
repeatedly checking.’”
MN 61
But Honorable Ānanda, what kind of bodily behavior is faulted by
sensible ascetics and brahmins?” This passage describes bad actions
from five perspectives, showing that the Buddha’s analysis of morality
was not based on a single factor.
Commentary by Ven. Sujato:
“Unskillful behavior.” “Unskillful” (akusala) is an unwholesome
quality of the mind, so this is the psychological dimension of
immorality. What is unskillful is that which leads to the result
opposite to that desired: we want happiness, but it leads to
suffering.
“But what kind of bodily behavior is unskillful?”
Commentary by Ven. Sujato:
“Blameworthy behavior.” “Blameworthy” (sāvajja) behavior is that which
makes one liable to criticism by others. This is the social dimension
of immorality.
“But what kind of bodily behavior is blameworthy?”
Commentary by Ven. Sujato:
“Hurtful behavior.” “Hurtful” (sabyābajjha) refers to the fact that
bad conduct is characterized by hurtful affliction both in its action
and its consequences.
“But what kind of bodily behavior is hurtful?”
Commentary by Ven. Sujato:
“Behavior that results in suffering.” “Results in suffering”
(dukkhavipāka) through painful kammic consequences in this life or the
future.
“But what kind of bodily behavior results in suffering?”
“Bodily behavior that leads to hurting yourself, hurting others, and
hurting both, Bad conduct does not just affect oneself, but
reverberates in space, by harming others, and in time, by promoting
the growth of even more unskillful qualities.and which makes
unskillful qualities grow while skillful qualities decline. That kind
of bodily behavior is faulted by sensible ascetics and brahmins.”
“But what kind of verbal behavior … mental behavior is faulted by
sensible ascetics and brahmins?” …
“Mental behavior that leads to hurting yourself, hurting others, and
hurting both, and which makes unskillful qualities grow while skillful
qualities decline. That kind of mental behavior is faulted by sensible
ascetics and brahmins.”
“Sir, does the Buddha praise giving up all these unskillful things?”
“Great king, the Realized One has given up all unskillful things and
possesses skillful things.”
MN 88
