We know that Parashurama killed his mother Renuka without a blink on his father's Jamadgani's command. But Manusmriti 2.145 says
In veneration, the Preceptor excels ten Sub-teachers; the Father a hundred preceptors, and the Mother a thousand Fathers.
So why did Parashurama obey his father's command ?
What Manu smriti stated is a general rule, when everything is going on in normal way.
However, in the instant case, Renuka got swayed away by the charms of the King Chitraratha's appearance and her mind got polluted. Hence, the general rule of Manu Smriti cannot be applied here and hence, the orders of father got precedence.
Now once upon a time, when her sons had gone out for the purpose of gathering fruits, Renuka who had a pure and austere life, went out to bathe. And, O king, while returning home, she happened to cast her glance towards the king of Martikavata, known by the name of Chitraratha.
The king was in the water with his wives, and wearing on his breast a lotus wreath, was engaged in sport. And beholding his magnificent form, Renuka was inspired with desire. And this unlawful desire she could not control, but became polluted within the water, and came back to the hermitage frightened at heart.
Her husband readily perceived what state she was in. And mighty and powerful and of a wrathful turn of mind, when he beheld that she had been giddy and that the lustre of chastity had abandoned her, he reproached her by crying out 'Fie!'
After reading Medhātithi’s commentary in Manusmriti 2.145 and 2.146, it seems that I might have an answer. Turns out Parashurama obeyed the orders because he didn't consider them to come from his father but from his "Guru" (i.e. person who taught him Vedas) who also happened to be his father.
From Manusmriti 2.146
Between the progenitor and the imparter of the veda, the imparter of the veda is the more venerable father; for the brāhmaṇa’s “birth” is the veda, eternally,—here as well as after death.—(146)
Now this seems to be contradicts 2.145. And Manusmriti 2.171 in support of 2.146 adds
They call the Teacher “father,” on account of his imparting the Veda. Before the tying of the girdle, the performance of no religious act is proper for him.
Now the transliterated 2.145 verse is
upādhyāyān daśācārya ācāryāṇāṃ śataṃ pitā | sahasraṃ tu pitṝn mātā gauraveṇātiricyate || 145 ||
And the seemingly contradiction with Manusmriti 2.146 is explained by Medhātithi as follows
Ācārya’ is not one who teaches; hence in the present verse the term stands for one who only performs the sacramental rites and teaches merely the rules of conduct;—Ācārya being one who makes one learn ā cāra....From all this it is clear that the superiority of the father here meant is only over that person who confers upon one only a slight benefit, who only performs the Initiatory Rite and teaches the Rules of Conduct, and does not do any teaching.
Thus dharma was upheld as Parashurama obeyed his Guru's command who is superior to parents
So why did Parashurama obey his father's command ?
This is answered in the Srimad Bhagavatam:
राम: सञ्चोदित: पित्रा भ्रातृन् मात्रा सहावधीत् । प्रभावज्ञो मुने: सम्यक् समाधेस्तपसश्च स: ॥ ६ ॥
Jamadagni then ordered his youngest son, Paraśurāma, to kill his brothers, who had disobeyed this order, and his mother, who had mentally committed adultery. Lord Paraśurāma, knowing the power of his father, who was practiced in meditation and austerity, killed his mother and brothers immediately.
उत्तस्थुस्ते कुशलिनो निद्रापाय इवाञ्जसा । पितुर्विद्वांस्तपोवीर्यं रामश्चक्रे सुहृद्वधम् ॥ ८ ॥
Thereafter, by the benediction of Jamadagni, Lord Paraśurāma’s mother and brothers immediately came alive and were very happy, as if awakened from sound sleep. Lord Paraśurāma had killed his relatives in accordance with his father’s order because he was fully aware of his father’s power, austerity and learning.
But nowhere does this Purana say that this killing was in accordance with dharma. In fact, the Mahabharata says this was done out of wrath:
And mighty and powerful and of a wrathful turn of mind...
Also, Apastamba sutra 1.1.2.19 says:
Ācārya adhīnaḥ syād anyatra patanīyebhyaḥ || 19 ||
- He shall obey his teacher, except [when ordered to commit] crimes which cause loss of caste.
Killing the mother is a mahapataka, which is worse than a sin which merely causes "loss of caste".
And Manusmriti 4.162 prohibits this killing:
He shall not injure his Preceptor, or Teacher or Father, or mother, or another elder, or Brāhmaṇas, or Cows, or any persons performing austerities.
Moreover, the Mahabharata passage here explicitly says that this killing was sinful:
sa vavre mātur utthānam asmṛtiṃ ca vadhasya vai | pāpena tena cāsparśaṃ bhrātṝṇāṃ prakṛtiṃ tathā ||
He chose for his mother to be brought back to life and forget the killing, non-contact with that sin (for himself), and for his brothers to be brought back to life.
