How do you begin care for an abusive, mentally ill parent that you are estranged from?

You don't. Don't open up this pandora's box of insanity. Mentally ill people will make you sick trying to cope with them.

You don't. It's not your problem. Let the state deal with and take care of him.

You call APS and report a vulnerable senior that has no person in his life because he is an abusive mentally ill man and then walk away.

If they call you, you tell them you ARE NOT his caregiver, NEVER WILL be his caregiver and for them to do WHATEVER they need to, you ARE NOT involved. Thank you very much, goodbye.

This is how you help him get the care he needs.

The 4 very wise people that posted comments below this are correct.
Please read the comments then reread them and take them to heart.
I am a FIRM believer that anyone that has been abused should NOT be a caregiver for the person that inflicted the abuse on them. (Nor for anyone that knew of the abuse but did nothing to stop it. )

Learn from my mistake, you don't.

APS. No hands on caregiving. Step WAY back.

This can only end badly if you get involved.

Wow! Thanks so much for the feedback. I reached out to a local geriatric care manager who gave the same advice. I passed her contact info on to my family members that are in contact with this elder and have the APS # on a sticky note on my fridge 🖤

I am going to say the same thing that Scampi and sp196902 said below....You don't. No child that was abused in any way from a parent should have to deal with their care as they get older.
Don't forget, you are estranged for a reason.
Instead call Adult Protective Services in your fathers area and report a vulnerable adult who shouldn't be living alone, and let them take over. He will become a ward of the state and they will make sure he's receiving proper care and get him placed in the appropriate facility if needed.
So now make sure that you're getting on with your healing so that you're not passing on the dysfunction that you grew up in.

You don't.
You report to APS as a Senior at Risk, and tell them that you do not wish to be involved and are estranged.
That simple.

I don’t recommend getting involved .
Call APS , let them handle it.