Violence Jack was an undercover cop who lost his memories because of the earthquake, just like Angel Cop and Mad Bull 34. Some reasons could be:
Ushio Mizuki, Jack's sidekick in Shin Violence Jack, is actually Tare Makimura, Miki's little brother from Devilman.When he sees Miki chained and with a mask, he grows terrified and runs away. When he finally stops, he realizes that he was about to remember something that was so frightening that he couldn't bring himself to remember it. What his memory is repressing is his horrible death at the end of Devilman, when the lynch mob decapitated him before they killed Miki. Likewise, when he runs back to help her ("even if it makes me remember"), he says to himself that he can't run away because otherwise he would go back to "being the cry-baby I was when I was little," which is an apt description of Tare in Devilman.
Violence Jack is Samurai Jack.