So this article makes it seem like he's got dementia and was under the spell of a delusion when shooting his partner & daughter. But it's written so vaguely that it's hard to know for sure. So I found another article that seems to be much more clear about what happened:

https://www.msn.com/en-us/news/crime/who-is-tony-louis-valez-father-sentenced-to-100-years-in-prison-for-killing-daughter-while-shooting-wife-and-son/ar-AA1jkw2q

TL;DR: 66-year-old man is in rough shape; has Hepatitis B, Emphysema, and COPD, and needs an oxygen tank. His partner (a much younger woman), their daughter together (8 years old), and his son from a previous marriage (18) all came to his house. The article doesn't say what happened in the conversation, but he apparently became enraged thinking they were trying to get him out of the house. My guess is that they were trying to get him to go into a care facility due to his many, many ailments.

Dude saw red, grabbed a gun, and started blasting. Shot his partner, then tried shooting his son, missed, and hit his 8-year-old daughter in the back as she was running away from him.

So it sounds much less like dementia (though that could still very well be a factor), and more like a miserable piece of shit reaching for a gun first to solve all of his problems.