My mother has had dementia for several years and it has taken me all that time and much therapy to actually have her admitted. She has her own room and it's a beautiful caring place, but now that it's done, she hates me and my brother was there causing trouble trying to get her out (though he has never done a thing to help with the situation at all) and I feel like I did take her life away from her. And I'm afraid I will feel guilty forever.
But if he does, she is HIS responsibility, not yours.
You may well feel regret for a long time, maybe even forever when you think about it. Certainly you regret that you had to do this. You regret that your mother is not happy. The situation is sad. Feeling sad makes sense.
But guilt? Your mother developed dementia. That is Not Your Fault. Your brother wasn't there for her. That is Not Your Fault. Her disease progressed to a point where she needed a higher level of care and 24 hour supervision. That is Not Your Fault. In her currently befuddled mind she is saying she hates you. That is Not Your Fault. This situation is not your fault. Give up the guilt. You haven't earned it.
Once your mother settles in (if not stirred up by your brother) she will probably forget that she "hated" you. And if she were in her right mind, she'd probably be appalled that she said that!
It may very well take you a long time to adjust to mom's new life - I had to make the decision to place my then 92 year old mom in memory care 15 months ago - I hate that I had to do it and it continues to be exhausting and stressful for me to be responsible for her
If you found a caring happy place for mom to live out her days then you are a hero - it may take her a very long time to adjust depending on where she is in the disease
As for your brother - was he trying to take her out for an outing or move her out?
Who has POA?
I'd seek legal advice about your brother trying to bother your mom to see what options you may have to prevent him from causing trouble. Some people just don't get it.
Sometimes, we are forced with a choice of keeping our elders happy or safe. Sometimes what they want is not a good choice. If we feel responsible for their safety, then we make the choice to get them into the level of care that they need, not what they want.
Guilt is for folks who did something wrong. Not you.
And by the way, it's her dementia that is robbing her of her life, not you. You got her to a safe harbor. She may eventually thank you for what you did.
Is she being seen by a geriatric psychiatrist for her depression and agitation? These conditions often accompany dementia (what I think of as "broken brain syndrome"). For some, meds for anxiety and depression have been real game changers; they won't fix the dementia, but they can fix some of the symptoms, sometimes.
Be well. Rest easy.