When faced with a situation like this, ask yourself "What is the harder thing to do"?
Whatever is the hardest to do, is the right thing to do.

I have had ~5 things like this come up in my life, chose the harder thing, and realized it was the right thing.
Go.

If you are questioning it, always go. You would know if you are not welcome and you wouldn't even ask or question it; you would just know not to go. It can really mean a lot to the family, even if you don't speak to them directly, to know your presence was there and you took the time to be there.

I have never regretted going to a Memorial service. There are several I regret not attending.

Go but don't approach

If you are available to attend, I would do so. The wife may be very appreciative. But if she still harbors some resentment that you cut ties, gracefully accept that bit of human nature.

We were friends with a couple for five years. We would vacation together and had many great times. The husband spiraled into a bad addiction. He had stints in jail and the wife stood by his side. We cut ties with them seven years ago. It was not pleasant.

We just learned the husband has died. He was in his 40's with kids.

Should I go to the memorial service? I feel conflicted since we parted on a bad note.

I am going to send a card.