I Made My Roommate's Coffee Taste Like Dirt On Purpose

So here's my dilemma. I'm not sure if I was acting like a real asshole or just getting back at someone who had it coming. Let me set the scene: I'm in graduate school, and sharing an apartment with two roommates, one of whom is Kate. She can be pretty mean sometimes, always making snarky comments about my personal hygiene or my studies. It's gotten to the point where every interaction feels like a battle. We've tried talking it out before, but she just brushes me off and keeps going as if nothing happened. The straw that broke the camel’s back was yesterday morning when I came home late from studying at the library and found out Kate had drunk my coffee without asking. Now, this isn't the first time it's happened; she does it quite often despite knowing how much I dislike sharing my coffee. She had left me a note saying 'oops,' but there were no real apologies or offers to replace what was gone. Today morning, something in me snapped. Instead of just buying new coffee like usual, I decided to mix things up for her. I took some of the ground coffee and sprinkled it with dirt from our garden pot and watered it a bit so it'd stick better. The idea was to make her drink 'her' coffee later and taste something nasty. I felt both sickened by my own actions but also strangely vindicated. Was what I did okay? Was I justified in retaliating, or am I just adding fuel to the fire of this toxic living situation? Should I just cut ties with Kate altogether even though we're practically roommates? Please let me know what you think. Comments and advice are welcome.