I noticed this parallel almost immediately after I saw Thranduil’s look of horror at the mention of a returning evil. And it made me think a bit, so I did some comparing and contrasting. While Legolas has not experienced what Thranduil went through (entirely) he’s been educated. It’s clear on his face when Gandalf mentions the Balrog. He’s scared. Legolas knows what a Balrog is, where they come from, and who they truly served. Thranduil has seen the horrors of Mordor and has undoubtedly retold his stories to his son. In a way I think he’s taught Legolas to be afraid, to know what he’s up against. To be smart in a sense, but not cowardly. It’s also chilling to see Elves, who are normally so serene and balanced, shaken up and fearful. It really gives the audience a perspective of what they are dealing with.