Perhaps Tolkien wanted to keep the theme of Morgoth being the fallen Vala who had to experience everything about the fana that he took. His rape of Arie was probably done with the same intentions as his lust for Luthien. They were both high, beautiful, and powerful, and "defeating" a creature such as that would have given him pleasure indeed.
I'm assuming that if Fingolfin and Thorondor could wound Morgoth's body, then he was probably physically "sound," to put it delicately. I can't see why he'd initially take on that "part" of the body, but he probably had his own, perverse, twisted, lunatic reasons.
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But Melkor also was there, and he came to the house of Fëanor, and there he slew Finwë King of the Noldor before his doors, and spilled the first blood in the Blessed Realm; for Finwë alone had not fled from the horror of the Dark.
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