Who Am I?

[Loslote]

Maedhros?

I tainted their white pride with red (Teleri ships stained with blood)
but stood aside while the red turned to black (stood aside when the ships were burned)

I'd have betrayed if not betrayed (he would have betrayed Morgoth at the peace meeting, only Morgoth beat him to it)
My yet another betrayal was with faith repaid (not sure about this one)

Sought the end but was paid in loss (the end of his vow; he lost the Silmarils)
Yet none shall claim in gratitude I was lacking (he was grateful to the Valar for defeating Morgoth)

My high hopes welded the children (not sure about welded, but the children could refer to Elros and Elrond)
adopted and born and the beloved youngest (again, he adopted Elros and Elrond)

In the darkness resigned plunged I, (he threw himself into a volcano)
My high delight set ablaze to grief and despair (he won the Silmaril, but no longer had a claim to it)