The more I think about this, the more I'm thinking this act is something he fantasized about and planned to do. I'm not sure he planned to it that particular night, but he did take the opportunity when he had it to fulfill his fantasy. And I think Ingrid was exactly the type of woman he fantasized about hurting. He could've picked up anyone rather easily - or even prostitutes - if all he wanted to do was kill and dismember. Instead, he killed and dismembered a really nice person, accomplished, beautiful, a mom, someone he knew and who knew him.
The fact he cleaned up well enough that her mom didn't notice what had happened in the house shows he knew what to do - probably from playing it out in his mind previously. It wasn't a moment of panic - it was methodical, even if he was intoxicated. Again, I don't think he planned this particular night, but had a running fantasy for a long time about hurting nice moms. Just my armchair opinion!
He was approaching middle-age and he could see his peers established in their lives, and he was past the age when being a drifter is okay. I think he couldn't handle life's responsibilities for whatever reason, used alcohol to self-medicate and sooth himself (which didn't work as it seem to bring out his demons instead of quieting them), and he developed these sick fantasies as a way to lash out at successful people.
TOTAL SPECULATION. I don't think he targeted Ingrid specifically to hurt her, but that he was attracted to her type (and who wouldn't be - she was beautiful in many ways), drawn to her, and then despised her and her family life when something triggered the feeling that he didn't measure up.
Perhaps a conversation happened on the date that triggered him. Maybe she didn't like his drinking, or asked him about his job or home....and it triggered that anger that made him lash out. And then his fantasy kicked in to destroy something beautiful, and he followed that script.
JMOPINION at the moment.