This is a terrible case, and I don't know all the facts here, but for a general answer to why killing pets when there's a suicide.
If the pets are old, and/or have health issues, and there are nobody trusted who can take care of them later, a person might prefer to kill/have a pet euthanised, rather than have the pet end up in a cage in shelter. A relative/friend left behind after a suicide understands what has happened, a pet would not know why "their human" is not coming back, and why they suddenly are in a strange place. If the pet is younger they could be adopted, but it's less likely with an older pet, most people want kittens and puppies, not fullgrown cats and dogs.
I bought a cat (3 - 4 years old) from a shelter, as I didn't want a kitten, and wanted a cat used being an indoor cat. The cat had been there for over a year, after her owner had died of old age. I remember how totally miserable she was when I saw her the first time, and it took a long time before she started trusting me. I knew I would never be able to even have her staying in a boarding cattery for even a week when going for a holiday. Thankfully I had friends helping me when needed, but they just caught glimpses of her when visiting.