[Eomer of the Rohirrim]

Or maybe Thranduil secretly feared that Legolas would usurp his throne, and so sent him to his likely death.

Think about what we know of the Mirkwood realm. Thranduil was very greedy. He wanted a lot of the riches from the Lonely Mountain despite having no decent claim on it. He was very prepared to sacrifice a lot of his fighters for a share of the spoils.

His rule was, to be blunt, a bit farcical. He held Bilbo and the Dwarves as prisoners in his domain. Gee, nice judge of character, old man. What's more, he was careless with them: assigning the task of supervising the prisoners to a few inebriated lackeys. The role of governer had obviously turned into a joke for the Elven-King.

What could cause such avarice, paranoia, carelessness, and incompetence? The appearance of the Necromancer? Or the problem of his brilliant and handsome son, ripe for Kingship?

You decide.