If a person has bad kamma as a result of the bad things done in the past, but he has a thorough understanding and diligently practise the noble eightfold path perfectly in the present life, can he achieve enlightenment in this present life, notwithstanding the yet-to-be manifested bad kamma?
It is attainable based on the description of Sutta SN 35.145 below.
SN 35.145
Kamma Sutta: Action
translated from the Pali by
Thanissaro Bhikkhu
© 1997–2012
"Monks, I will teach you new & old kamma, the cessation of kamma, and the path of practice leading to the cessation of kamma. Listen and pay close attention. I will speak.
"Now what, monks, is old kamma? The eye is to be seen as old kamma, fabricated & willed, capable of being felt. The ear... The nose... The tongue... The body... The intellect is to be seen as old kamma, fabricated & willed, capable of being felt. This is called old kamma.
"And what is new kamma? Whatever kamma one does now with the body, with speech, or with the intellect: This is called new kamma.
"And what is the cessation of kamma? Whoever touches the release that comes from the cessation of bodily kamma, verbal kamma, & mental kamma: This is called the cessation of kamma.
"And what is the path of practice leading to the cessation of kamma? Just this noble eightfold path: right view, right resolve, right speech, right action, right livelihood, right effort, right mindfulness, right concentration. This is called the path of practice leading to the cessation of kamma.
"So, monks, I have taught you new & old kamma, the cessation of kamma, and the path of practice leading to the cessation of kamma. Whatever a teacher should do — seeking the welfare of his disciples, out of sympathy for them — that have I done for you. Over there are the roots of trees; over there, empty dwellings. Practice jhana, monks. Don't be heedless. Don't later fall into regret. This is our message to you."