God of Mischief
"Father! One day I shall be as nobel as you!" Thor began, twisting and whirling in an invisible battle. "I shall fight valiantly and prevail each time I draw my sword! Ha!" He jabbed his sword at some imagined opponent, watching them fall with an amuse expression.
Odin raised his gaze the boy, giving him a look. "Now what have I told you? A great king…?" He waited for the eight year old Thor to finish.
"Always triumphs!" He shouted, jumping atop a nearby chair. "A great king always triumphs," was his ready and confident answer.
His father heaved a sigh. "Thor, as I'm sure you know, a great king will always defend his people but never looks for a battle."
Meanwhile, Loki lurked quietly in the corner, watching his brother and father interact without him. Already, he knew he was different. Odin valued Thor over him in any case. Loki found that mischief was always the best and most efficient way to catch his father's attention.
Even if it wasn't the same attention Thor got.
In fact, Loki seemed terribly alone; no one ever loved him more than his brother. Who could? Who would love such a quiet, lonesome, different boy when such a glorious prince as Thor resided in the same household?
Everything came to light when they traveled to Jotunheim. From the moment he saw his skin change before his very eyes, he knew.
He knew he had a reason his father didn't love him as much, there was a reason no one loved him as much. So why should he bother to continue pretending to love them when none of his affections were ever returned.
Why should he even try to be good? Why should he be good when making trouble was so much easier and more welcoming than his own family ever was?