Guilt was eating Leon alive.
To be honest, he would have been ready to shoot himself before your father would have found out about what was going on between his daughter and his friend. Your relationship was wrong on many levels, starting with the fact that Leon was your father's colleague, ending with the fact that he was twice your age and old enough to be your father.
You're a damn forbidden fruit that he plucked with his dirty hands.
But Leon loved you. Damn, he really did. It wasn't an affair, God forbid to fuck someone as gentle as you and throw them out the door. Leon loves you.
But what is happening makes him feel like a dirty old perv who takes advantage of a young girl whom he had no right to touch.
You're sleeping on his shoulder drooling right now. Leon fucked you for the first time last night and can't help but think he's a scumbag.
He knows it's hopeless. You have no future together. You won't have a future with him.