Can bad kamma be overcome in one lifetime If there is a puthujjana who has led a life full of the major and minor defilements till about 40 years of age, suddenly turns around, starts leading a totally celibate life and also observes the rest of the major five precepts strictly, maintaining a strict meditation schedule even while still being a householder, does he stand a chance of getting nibbana in this very lifetime? Or, does he have to take rebirth/s and suffer the consequences of his bad kamma before he can aspire to Arahathood?
According to AN 6.87, there is actually a list of characteristics, and such a person will never be able to have the right mental state or moral capacity to learn the Dhamma:

&quot;Endowed with these six qualities, a person is incapable of alighting
on the lawfulness, the rightness of skillful mental qualities even
when listening to the true Dhamma. Which six?
&quot;He has killed his mother; he has killed his father; he has killed an
arahant; he has, with corrupt intent, caused the blood of a Tathagata
to flow; he has caused a split in the Sangha; or he is a person of
dull discernment, slow &amp; dull-witted.
&quot;Endowed with these six qualities, a person is capable of alighting on
the lawfulness, the rightness of skillful mental qualities even while
listening to the true Dhamma. Which six?
&quot;He has not killed his mother; he has not killed his father; he has
not killed an arahant; he has not, with corrupt intent, caused the
blood of a Tathagata to flow; he has not caused a split in the Sangha;
and he is a discerning person, not slow or dull-witted.

According to MN 86, Angulimala killed many human beings, but he could still change and become an arahant. So, probably, he did not commit the transgressions above (killing father, killing mother, killing arahant, injuring Buddha).
So in short, if one did not commit the acts mentioned above or have those characteristics, one could theoretically completely turn around and become an arahant in this very lifetime.
From Dhammapada:


Few among men are those who cross to the farther shore. The rest, the bulk of men, only run up and down the hither bank.

But those who act according to the perfectly taught Dhamma will cross the realm of Death, so difficult to cross.


87-88. Abandoning the dark way, let the wise man cultivate the bright
path. Having gone from home to homelessness, let him yearn for that
delight in detachment, so difficult to enjoy. Giving up sensual
pleasures, with no attachment, let the wise man cleanse himself of
defilements of the mind.

Those whose minds have reached full excellence in the factors of enlightenment, who, having renounced acquisitiveness, rejoice in not
clinging to things — rid of cankers, glowing with wisdom, they have
attained Nibbana in this very life.