Follow my logic here. Jack's personality seems like a combination of the two, and in addition, he has traits that wouldn't make sense if he was purely Akira or purely Amon.
- Satan is actually overjoyed and obbessed with him, which Neo Devilman and other media have established Satan cared about ONLY Akira Fudoh and couldn't give half a crap about Amon.
- He seems to care for Miki instinctively, another Akira thing
- His speech to Mondo during the El Dorado arc mirrors Akira's various "The Reason You Suck" Speech that he would give out to various demons.
- His interactions with children is much nicer and less murderous or simply annoyed as Amon's interactions with them in the OV As and Amon's manga.
- Looks like an adult, muscled Akira.
- Willingly gives his powers to those who need it twice
- Has Amon's markings from Demon Knight in Shin Violence Jack,
- Actually gives a crap about Siren
- His speech on Might Makes Right in the Slums arc
- His apparent knowledge on the goings on of The Skull King.
These are massive contradictions.....unless they aren't. My theory is Violence Jack is the reincarnation of all of Akira and Amon's good traits (Akira's humanity, curiousity and morality, Amon's powers, approach to combat, and fighting philosophy.) Skull King is the incarnation of what was left, hence why all they feel is despair and disappointment, even in acts of violence.
- It is also possible that after Akira disappears in the ball of light with Satan at the end of Violence Jack Akira's half disappeared with him, leaving behind only his bad emotions as Shin Violence Jack takes place afterwards. Jack is shown to still exist at the end of Violence Jack. He is now treated as Amon by his fellow demons and not as Devilman, meaning he potentially has inherited enough good qualities to fight for humanity himself just as Akira did. Xenon, who was still around at the end, likely takes advantage of the situation to make Akira's dark side into the Skull King. Himself being Skull Castle and the "Dark Side" of Satan due to them fusing together in ages past.