People do feel like that sometimes if a parent or any relative is very elderly and ill; death is then a blessed relief. It doesn't mean they are not loved - the person themselves may wish their life to be over.
I have a cousin who has just turned 90. She was fine up until a couple of years ago, then everything changed. I won't go into details but she developed dementia and really was not safe, she caused no end of problems for neighbours and was going out at all hours. She had a dreadful row with another cousin, even throwing her out of the house, who was deeply hurt at the time but did after a while realise the cause. She tried phoning and still delivered birthday and Christmas cards through the letterbox but that was all (she died last year at 84 but that was quick, it was quite a shock).
Cousin (90) had carers going in to make sure she was washed, dressed and fed. I last spoke to her before Christmas, she phoned me and wanted to know if I had received her card, had she sent it to the right address (I moved last year). She sounded fine then. We chatted about her family. A couple of weeks later I phoned her and a carer was there, she gave the phone to the carer and said, crossly, she didn't know who I was or what I was talking about.
She has three children, all now retired people. They were always a close family, very loving, but two of them don't live near though they saw her often and did anything for her. Nobody could watch her 24/7 so early this year she went into a care home, somewhere that is easy for them to get to.
Her eldest son sent me a photo of her a couple of weeks ago, taken on her birthday, with birthday cake. I was really upset by it, she looked like a cadaver.
We were not close (she was close to my mum), but she was always in my life and I loved her children who are all a few years younger than me. I was her bridesmaid when I was little.
I am pretty sure my cousin's children will consider their mother's death a happy release and I know they love her very much. She would not be in a care home had she been manageable at home, I am sure of that. However dementia is a whole different ball game if the person concerned becomes angry and is physically well enough to be going out and creating havoc (some are not like that, they just go quiet and take to bed).
My father in law had a very painful illness which resulted in his death in his seventies and that was a release for him but he was loved.
It's not always bad to not be sorry, or even to be glad, when a parent dies.
I am 75 and doing my best to keep healthy, independent and not be difficult :-), but nobody knows what is around the corner.