His parents we're just as bad

Indeed but there was a great scene when he met that trailer pair, the woman said you look like you were loved, you have to give them that which is evidenced in the end when the dad has a nervous breakdown in the middle of the road. They were just as unprepared for parenting as Alex was for his choice of life.

Well then take a hike.

To clarify (without justifying anything that Walt McCandless did), there was nothing "secret" about his two lives. He kept his two families' children in touch with each other via regular vacations and trips which Marcia's children remember with mixed emotions. All the while he was defaulting on support payments. However, his two families knew about each other and the children were kept in touch.

So, the existence of Walt's other family was not a secret, but the secret that purportedly devastated Chris was finding out, in his late teens, that his father and his mother were not married at the time of his birth. This apparently was devastating for him; he had previously had a pretty good relationship with his father, according to acquaintances and friends from that time. But, learning that he was legally a "bastard" at birth had a catastrophic effect, apparently. It was this deception, more than the "two families" issue, that alienated him. He saw his mother as an enabler, if not as the culprit.

Chris's half-sisters speak movingly about their experiences and their knowledge of Chris in a documentary entitled (IIRC) Back to the Wild. They comment on how Chris seemed to be bothered, from an early age, by the financial inequities between the two families -- Marcia's children struggling in near-poverty while he and his sister lived a life of privilege and affluence. Both families reflect on how Walt's abusive behavior affected them. He beat both wives (though not the children), but the physical spousal violence was psychologically damaging to the kids, in ways not appreciated at the time.