Continuity errors in Tolkien

[Finwe]

Yes, but Legolas said "Morgoth," not "Melkor." The Elves have a loooong history of enmity with Morgoth, which is why they don't address him by his "real" name.

Morgoth was given the name "Morgoth" by Fëanor, because he killed dear Fe-Fe's father, Finwë (yep, I got my skull bashed in by a Dark Lord's mace. Next time, remind me to invest in a better security system.) and stole his Silmarilli. Since that didn't sit too well with Fe-Fe, Morgoth got declared the "Dark Enemy" (hence the name Morgoth, which means "Dark Enemy") and Fëanor and his sons said an oath that they would not rest until all three Silmarilli were back in their possession, and that they would kick the butt of whoever withheld any of the Silmarilli from them.

For an Elf to call Morgoth by his other name (*shudder shudder* Melkor *shudder shudder*) would be like saying that he wasn't the enemy of the Elves at all. The name "Melkor" means "One who arises in might," a rather positive name, to be sure. No self-respecting Elf would praise Morgoth like that, and call him by his real name.