It does appear that Westman regretted his transition and his mother wasn't supportive of it (even though she assisted in the name change). Due to language in quotes from Westman's manifesto, I can't show the NY Post link, but here are some excerpts, including the headline:
Minneapolis gunman Robin Westman blamed massacre on mom warning him not to change gender and discouraged people from letting their kids transition
Deranged Minneapolis gunman Robin Westman blamed his mother for his decision to kill “so many people†at a Catholic school after she warned him that he would regret transitioning to female  while in the next line in his manifesto, he discouraged people from letting their kids change gender.
In his disturbed manifesto, Westman, 23, claimed to be anxious about a visit from his mother in an entry dated July 1, where he recalled her concerns about him transitioning.
“Your words, mother, made me stay in my discomfort unable to ask for help to avoid admitting defeat. You were right mama, but the way you handled it led me to wanting to kill so so many people,†he wrote.
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In the rambling writings, the crazed gunman also appeared to blame transitioning and smoking marijuana for his violence.
“Gender and weed (messed) up my head,†he claimed. “I wish I never tried experimenting with either. Don’t let your kids smoke weed or change gender until they are like seventeen.â€Â
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“When I was first out to my mother, she was VERY antagonistic. She really made me hate myself and think I will never be good enough,†Westman wrote.
“I remember one day, she said something like, ‘In the future you will look back and feel ridiculous about who you feel like you are inside. You will regret this.’
“Well guess what mom?! I regret being born,†the gunman added.
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Despite the antagonistic feelings displayed in the manifesto, Westman penned a separate letter to his family and friends telling them how much he loved them and how his parents shouldn’t blame themselves with how he turned out.
“You did not fail me, you gave me so much. I truly appreciate the love you have given me,†Westman said to his parents. “I feel I was raised to be a good person.
“I’ve kept those traits of empathy, self-sacrifice, and good character. Please do not think you have failed as parents,†he added.
He admitted in a series of disturbing online videos that he regretted being brainwashed into “being trans,†and planned to cut his long hair before unleashing his carnage, in which he planned to kill far more kids.
Westman also wrote that he could neither stop smoking  which he suspected without apparent evidence had given him cancer  nor the attack that claimed two lives and left 18 others wounded at the Annunciation Church on Wednesday.
“I wish I could stop vaping but I can’t! If I could stop vaping, I could stop myself from doing this attack, but I just don’t want to. Stopping vaping will not get rid of my cancer, nor will it make these fantasies go away.â€Â