Dr.Julien's mental health declined faster than his physical health did. The fact that Zane was the only person he talked to for years did a number on Julien. What made it worse was that Julien knew zane wasn't a real person, so even if he treated zane like his son, he knew he was just regurgitating information into a machine to teach it how to live. And that drove him to paranoid and depressed madness.
Do you think Zane keeps himself up at night thinking of him? Thinking of how badly he let his own fathers mental health get. Then blaming himself for Echo becoming mr.E because if Zane helped Julien, maybe Echo wouldn't be neglected like Zane was. But there was nothing Zane could've done to help Julien because he was just too young, he didn't know how emotions worked let alone an old mans mental health after isolation for god knows how long. The only thing Zane could've done was watch as Julien's mental and physical health declined while the small house they shared rotted away. Bugs everywhere, mold growing on dirty plates Zane was too busy with other chores to clean, Julien's workshop covered in splats of oil and ripped up sketches from his past outbursts, no place for someone so young to grow up in. Let alone have to worry about if his father was going to lose his marbles at the drop of a dime.
Then zane finds himself living with 10 other people by DR, and the moment someone maybe forgets to do the dishes one day or leaves a mess somewhere, Zane finds himself back in the old house him and Julien lived in. And its impossible to get him out of the spiral once hes reminded of it. He'll dwell about his memories for days, if he doesn't distract himself quick enough he'll lunge into a panic attack over it.
You could've been a better son. You were built to protect those who cannot protect themselves but you couldn't even protect your own father from himself? How do you expect to protect everyone else if you leave your own father to rot?