Curate, connect, and discover
From what most Tekken enjoyers understand is that the Devil gene was retconned. But what if there was a way to watch all endings and past revelations without getting confused? Which brings my presentation.
How the Devil Gene in canon still works after Tekken 7
To be clear, I haven't refreshed my memory of what went down in the storyline but I remember the basics, so feel free to call out any incorrectness. Most of this is just a ramble and I wanted to show it to anybody who would read it, along with using a bunch of gifs for artistic reasons.
When Kazuya was a small boy his father (Heihachi) threw him off a cliff, in Tekken 7 it is because Kazuya tried to fight him after Heihachi killed Kazumi, his mother. The reason Heihachi killed her is because he found out she came from a family of devil genes (or something).
I don't think Kazuya was raised to know what the Devil gene was, so when Devil made himself present he naturally made a deal with Kazuya out of instinct of knowing what itself was but not the family history. Kind of like a kitten having the same tendencies as its parents but was taken from the litter and mother it belonged to. I think you guys know what I mean here.
I theorise that the gene itself isn't present until something tragic and emotionally distressing activates it. At least that may be the case for those with it who go untrained not knowing that they have it, I don't know how it must've been for Kazumi. Devil Jin was activated through stress, and whilst Jin was in his teens, while Devil K grew with Kazuya.
Dude's mum just disappeared and the person that's supposed to protect him betrays him with the actual intent to harm. I think Heihachi throwing Kazuya was kind of a disciplinary thing, because Kazuya still grew up with Heihachi, while Jin was left for dead. But Kazuya was 8-10 (poor memory) when he got thrown off, so his emotional/mental tolerance was... lesser. Not to say Jin losing his mum at an older age wasn't just as traumatic, but teenagers and children are different. Jun's presence also must also have something to do with it, as Devil Jin tries to resist her good powers.
Devil Jin must've been waiting to appear for quite some time, in the back of Jin's mind watching everything. What we know of Devil Jin is that he's an absolute menace to society and a typical delinquent teenager. He's very playful in comparison to Devil Kazuya's maturity. He takes his time being a menace, knowing Jin would take control eventually. TTT2's special win poses between Devil K/Kazuya and Devil J make Devil K/Kazuya look a disapproving father. From DJ firing his laser out of control and flying away, with DK either crossing his arms or showing no reaction.
In Tekken 4 the Devil in Kazuya says that he lost a part of himself and wants to take it back. In a way, when you put in the whole genetic thing he's kind of not wrong. Devil K is acknowledging that he has a son but not to the full grasp that he should. We all know how DNA works, reproduction, ect.
Devil K however believes that 'part of himself' should be taken back, not getting that the gene is in Jin's blood. While I do believe Jun was protecting Jin, I don't think that's why his body rejected the gene getting taken away. Now, as seen with Kazuya's TTT2 ending he can absorb the evil off others for his own gain which is what he did to Unknown. Thing is, Unknown is more of a possession rather than an 'in the blood' situation. Sure TTT2 isn't canon, but it has shown 'what if' scenarios. Not only that, but we see Zafrina use Azazel's ball thing for her own use.
So the Devil gene is still a thing, and it started out as just that, a gene. They can take power away from other forms of evil, but not their own family members. That may have been why Kazumi wanted Kazuya killed, because Devil couldn't be taken away from him, and she's no Jun Kazama or Angel so it's not like she could take it away from him. Not to defend her or anything, just saying.