SERIOUS ETHICAL question about what i've done

So during last spring semester at college I lived in a frathouse with
the majority of my fellow brothers. On of the brothers is a complete
piece of trash. I mean real scum. This guy would skim hundreds of
dollars out of our dues fund to furnish his room and pay for his share
of the rent. He is also disgustingly fat and smells like horse crap,
but that is another matter. I'm just trying to set the stage for what
a dishonest, self-absorbed disgusting freak this guy is. Anyway, the
only thing he liked more then eating and being an asshole and watching
TV was his fishtank. From what I could estimate it was about an 80
gallon thing. Huge. He had all sorts of fish in it which he always
boasted about paying lots of money for. I always thought to myself
"yeah, money you imbezzled from the house fund." There were beautiful
fish of all sizes in it and I must admit for as big a jerk as he was,
he did a good job setting it up. Anyway, to get to the point, one day
I got fed up with this asshole and when he wasnt there I snuck into
his room and poured an entire bottle of aristocrat vodka into the
tank. Needless to say, a holocaust ensued. By the next night, EVERY
SINGLE fish in the tank had been wiped out. There were even some
around the tank on the floor which means they must have jump to their
doom in order to escape the agony of the toxic water. You couldn't
even see into the tank there were so many floaters. Let me tell you
how ****ed this guy was, he ended up fistfighting like half the
brotherhood in his rage. He never suspected me for some reason tho so
i got away SCOT-FREE. My question is, is there an ethical concern for
massacring so many fish? Or was I justified because this guy was such
a waste of life? At anyrate, the plan worked perfect, and I slept
better then I have in a long time.

-Slick