Good angel?

[BG-57]

Edea knew that she wanted to give up her powers, she told Squall that.

Then why does this dialog
"in order to die, you need to give up your powers, getting killed, praying, blah blah...." exist? I mean, if special people want to last forever, then this dialog would have never existed.

You mean this dialogue?

[The screen fades into the lighthouse orphanage, intact because it
is in the past.]

Edea: "Squall!"

Squall: (...Matron.)

Edea: "Excuse me. Have you seen a little boy?"

Squall: "You don't have to worry. The boy won't go anywhere."

Edea: "I think so, too. Poor thing..."

[A fatally-wounded Ultimecia appears, and staggers towards the two.]

Squall: "...You're alive!?"

Edea: "...The sorceress?"

Squall: "Yes, Matron. We had defeated her... Matron, stand back."

Edea: "It's ok. There's no more need to fight. That sorceress is just
looking for someone to pass her powers on to. In order to die in
peace, a sorceress must be free of all her powers. I know...for
I am one, too. I shall take that sorceress' powers. I don't
want one of the children to become one.

Ultimecia: "I...can't...disappear yet."

[Ultimecia passes her powers onto Edea and dies, sinking away.]

Ultimecia doesn't sound suicidal here. She even denies her impending demise. And this business occurs after she is defeated and badly wounded.

It seems more likely that she went into the battle intending to become all powerful. Her actions certainly suggest this course of action.