2022 marked the 20th anniversary of one of the most significant set of films made in Hong Kong. That said, the most important yet most overlooked spiritual takeaways from the ‘Infernal Affairs’ (无间道) trilogy are the opening quotes featured, which are more profound than they seem, in foreshadowing what to come for the antagonist(s). The movies are thus a trio of spiritual cautionary tales interwoven as one layered story.
‘无间道’ usually means the worst hell realm (地狱道), but here, it also refers to the evil path being taken to this hell. Below are their title cards in Chinese, and as translated, more accurately than in the default English subtitles. Although some liners are supposed to be from Buddhist scriptures, as exact corresponding wordings are not found, (perhaps due to misquoting or paraphrasing), here they are, without attributions, (as they are still valid teachings), with notes added.
Infernal Affairs [Uninterrupted Path] 
‘The most extreme of the Eight Great Hells, is called the Uninterrupted Hell, because of its meaning of having uninterrupted encountering and receiving of great suffering, thus having this name… Receiving that body uninterrupted, forever [i.e. for a very long time] not dying, with lifespan long thus, is the great misfortune of the Uninterrupted Hell.’
[Note 1: The Eight Great Hells (八大地狱) are (i) Great Hell Of Waiting For Revival (等活大地狱: Sañjīva Mahā Nāraka), with suffering for 1 trillion 665.3 billion (1兆6653亿) years. (ii) Great Hell Of Black Strings (黑绳大地狱: Kālasūtra Mahā Nāraka), with suffering 10 times of the above hell, for 13 trillion 322.5 billion (13兆3225亿) years. (iii) Great Hell Of All Closing (众合大地狱: Saṃghāta Mahā Nāraka), with suffering 10 times of the above hell, for 106 trillion 580 billion (106兆5800亿) years. (iv) Great Hell Of Wailing (叫唤大地狱: Raurava Mahā Nāraka), with suffering 10 times of the above hell, for 852 trillion 640 billion (852兆6400亿) years. (v) Great Hell Of Great Wailing (大叫唤大地狱: Mahāraurava Mahā Nāraka), with suffering 10 times of the above hell, for 6821 trillion 120 billion (6821兆1200亿) years.
(vi) Great Hell Of Scorching Heat (焦热大地狱: Tapana Mahā Nāraka), with suffering 10 times of the above 5 hells combined, for 5 koṭi 4568 trillion 960 billion (5京4568兆9600亿) years. (vii) Great Hell Of Great Scorching Heat (大焦热大地狱: Pratāpana Mahā Nāraka), with suffering 10 times of the above 6 hells combined, for half an intermediate kalpa (半中劫), which is 167.98 million (1.6798亿) years. (viii) Great Hell Of Non-Interruption (阿鼻大地狱/无间大地狱: Avīci Mahā Nāraka). With suffering of the above 7 hells combined with all suffering in other places as 1 part of suffering, this hell is with suffering 1000 times of this part, for 1 intermediate kalpa (1中劫), which is 335.96 million (3.3596亿) years.]
[Note 2: The Uninterrupted Hell (无间地狱) is thus the hell with the worst suffering for the extremely evil, who were not repentant in time in their previous lives. As committing of one or more of the Five Heinous Transgressions (五逆罪) is not the only way to fall into this hell, with other equivalently extreme evils also ‘qualifying’, the point is to not be evil at all, to avoid falling into any hell, even if it is a lesser hell.]
[Note 3: The Five Heinous Transgressions are (i) killing of father (杀父), (ii) killing of mother (杀母), (iii) killing of Arhat (杀阿罗汉), (iv) causing Buddha’s body to bleed (出佛身血), and (v) breaking up the harmonious Saṃgha (破和合僧).]
Infernal Affairs [Uninterrupted Path] 2
‘That “A” is said to be “without”, and that “Vīci” is named as “interruption”. [Avīci (Uninterrupted) Hell (阿鼻[无间]地狱)] is without time’s interruption, is without space’s interruption, and is the realm of immeasurable receiving of karmic retribution. The uninterrupted has three kinds, with time uninterrupted, space uninterrupted, and receiving of karmic [retribution] uninterrupted. Those who commit heinous transgressions will long fall into this realm, to utmost receive the uninterrupted of ultimate [suffering].’
[Note 4: The Uninterrupted Hell is with long suffering, that is still not infinite in time. This is so as although sentient beings can be very evil, none can be infinitely evil, to thus create infinite evil karma, to deserve infinite retribution forever.]
Infernal Affairs [Uninterrupted Path] 3: [The Ultimately Uninterrupted]
‘Such kind of [extremely evil] beings, will fall into the Uninterrupted Hell, for ten million koṭis of kalpas, with this continuing uninterrupted, seeking exit without a date.’
[Note 5: The Uninterrupted Hell is thus with extremely long suffering that will still eventually end, although it is so long that in the midst of it, it feels like it will last forever. Such is the nature of truly and fully experiencing the hellish.]