Why do I want to hurt someone weaker than myself? i won't, but I have the desire.

I'm a girl at college. There's this boy I've met at college. He's brilliant, absolutely adorable, and the sweetest human being I've ever met. I mean, this kid will run himself ragged and put himself at risk to help random people. He won't kill insects indoors and would rather spend an hour getting them outside. He was brutally beaten and when he found that his attacker was poor and hungry, he bought him food and left him what little money he had anonymously. He is such a little sweetheart. He's really underweight; extremely underweight. He's frail, fragile and weak. I like him a lot, but I have these destructive desires to hurt him. Of course I don't act on them, but they are there. He's so weak physically and such a pacifistic little sweetheart that I know I could physically dominate him if I wanted to. I think he's really cute; he has big brown eyes and dark, longer straightened hair that covers his eyes. He's a little cutie, but it's that innocent, fragile look that makes me have these feelings. I care about this kid and he cares about me. I have no intention of acting on these desires, but I have them anyway. Does anyone have any thoughts?