The god of death - Yama and the first Jnani/SELF REALISED among humans - Manu were the same person

Manu (Sanskrit: मनु) is a term found with various meanings in the Rig veda, brAhmanAs, Puranas, etc. In Rig veda, it refers to the first man (progenitor of humanity). The Sanskrit term for 'human', मनुष्य (manuṣya) or मानव (mānava) means 'of Manu' or 'children of Manu'.In later texts, Manu is the title or name of fourteen rulers of earth, or alternatively as the head of dynasties that begin with each cyclic kalpa (aeon) when the universe is born anew.

Yama (यम), also known as Kala, and Dharmaraja is the Hindu god of death and justice, responsible for the dispensation of law and punishment of sinners in his abode, Yamapuri.

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Rigveda 4.26.4 mentions that a divine bird offered Soma (amrita) to the  Manu. 

प्र सु ष विभ्यो मरुतो विरस्तु प्र श्येनः श्येनेभ्य आशुपत्वा । 

अचक्रया यत्स्वधया सुपर्णो हव्यं भरन्मनवे देवजुष्टम् ॥

Meaning

Let that bird stand out from (all other) birds, o Maruts, the swift-flying falcon from (all other) falcons, because the fine-feathered one, with his independent power that needed no (chariot-)wheel, brought to Manu the oblation that is pleasing to the gods.

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Apart from the history of Manu described in the Puranas, Manu also means a wise man, who has reached spiritual heights. It means that the one who attained the level of Jnani for the first time among human beings is Manu. 

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Rigveda 10.14.2 states that the first discoverer of the spiritual path is Yama. It is said that the sages, who came later, traveled on that path. 

यमो नो गातुं प्रथमो विवेद नैषा गव्यूतिरपभर्तवा उ । 

यत्रा नः पूर्वे पितरः परेयुरेना जज्ञानाः पथ्या अनु स्वाः ॥

Meaning

Yama first found the way for us: this pasture-land is not to be taken away.  (The way) on which our ancient forefathers departed, along that (do) those who have since been born (follow) along their own paths.

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This means that Yama, who is described as the god of death in later texts Upanishads/Puranas, must have been the  Manu himself,  who attained the level of Jnani/SELF REALISED among humans, for the first time. 

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That is why in the Kathopanishad (कठोपनिषद्), Yama was described as the knower of what lies beyond death and teaches that knowledge to Nachiketa.

Katha Upanishad 

1.1.20

येयं प्रेते विचिकित्सा मनुष्येऽस्तीत्येके नायमस्तीति चैके ।
एतद्विद्यामनुशिष्टस्त्वयाऽहं वराणामेष वरस्तृतीयः ॥ २० ॥

(Nachiketas said) This well-known doubt as to what becomes of a man after death,—some say he is and some, he is not,—I shall know being taught by thee. This boon is the third of the boons.

1.1.21

देवैरत्रापि विचिकित्सितं पुरा न हि सुविज्ञेयमणुरेष धर्मः ।
अन्यं वरं नचिकेतो वृणीष्व मा मोपरोत्सीरति मा सृजैनम् ॥ २१ ॥

Here, even the gods of yore had doubt. Indeed it is not easy to know—subtle is this matter—Oh, Nachiketas, ask for some other boon. Press not this on me; give this up for me.

1.1.22

देवैरत्रापि विचिकित्सितं किलत्वं च मृत्यो यन्न सुज्ञेयमात्थ ।
वक्ता चास्य त्वादृगन्यो न लभ्यो नान्यो वरस्तुल्य एतस्य कश्चित् ॥ २२ ॥

Thou sayest, Oh Death, that even the gods had doubts here and that this is not easy to know. None other like thee, who could tell of this, can be found; no other boon can at all equal this.