First off, I confess to not being familiar with the Peake work, so I cannot speak of the parallels between that story and Túrin's.
Túrin had the will of a hateful divine being working against him, but in the end, it was his personal shortcomings that did him in.
The will of Morgoth may have entered into Menegroth, working through Saeros to drive Túrin out, but it was Túrin's pride that did not allow him to return there when told he had been pardoned in the death of Saeros and would be welcomed back in honour.
In Nargothrond, his pride and love of valour in arms led him to have the bridge over the Narog built, ultimately leading to the fall of Nargothrond and the death of Finduilas.
It was the latter, I think, that was really the nail in the coffin, so to speak. If Finduilas had not been captured, or if Túrin had been able to save her, he would have saved himself. He was told as much by the dying Gwindor.
Quote:
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Haste thee to Nargothrond, and save Finduilas. And this last I say to thee: she alone stands between thee and thy doom. If thou fail her, it shall not fail to find thee.
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Silm Of Túrin Turambar
I think the grave of Finduilas,
Haudh-en-Elleth, figured into his acceptance and ultimate love of Níniel.
Quote:
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Haudh-en-Elleth! From the green mound she came! Is that a sign, and how shall I read it?
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UT Narn I Hîn Húrin
So it seems to me he was lost after Finduilas died, and unable to escape his doom.
Morgoth may have led Túrin to a path of destruction, but it was Túrin who chose to follow it.