Is Heather really. . . (SH1 and 3 spoiler)

[alone in the town]

Question: Why does Incubator/Holy Woman in White try to kill you in Bad/Bad+ scenarios but help you escape and give you a baby in Good/Good+

Because the Holy Woman in White gives Harry the infant we later know as Heather, and as we all know, Heather will one day be used as a vessel used to allow God to be resurrected. It's a contingency plan and a damned wicked deception.

[AuraTwilight]

Question: Why does Incubator/Holy Woman in White try to kill you in Bad/Bad+ scenarios but help you escape and give you a baby in Good/Good+

Th in the Bad endings, the Woman in White (God) is doing her job of, you know, killing everyone. But in the Good endings, you kill her alternate form, and so she uses a last ditch effort to take the Woman in White form and give Harry a baby in an act of self-preservation (and because, as God, it might have instincts to console Harry's heart while it can while it's at it).

This doesn't work in the Bad endings because, well, Harry's not going to trust the Woman in White after it tried to kill him, no matter how beautiful and angelic-looking she is, and she might be too damaged to even try. However, she'll still try and do what she can for Harry by letting Cheryl speak to him one last time.

[swordandshield]

Think of Alessa and Cheryl as identical twins. Both come from the same source, but they have grown into two different people. The same with Heather.

As quoted, Alessa and Cheryl are twins sort of like two fruits coming from the same root but branched off into two seperate fruits. They both have to co-exist and not become bad fruits (spoil) because that tree that they were on will wither and fade away and Heather was the fertilizer used as a last ditch effort to restore the tree so that it can remain docile and pure. It is the same premise on them with the god.