I've killed a couple of fish (literally two) when my dislike of them caused me to go into denial about their health.
Example: An overstocked tank had an outbreak of ich and one of the newer fish I wasn't attached to was not doing well (swimming upside down, continuing to have ich when all the other fish were responding to treatment, etc). So I "quarantined" him in a 5 gallon tank with no heater, assuming he would be dead by morning.
Two days later he is refusing to die. Perhaps even getting better, but I leave him in my poorly set up QT tank because I don't want him back in the main tank.
Eventually he dies, probably from the cold water.
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I feel like I killed a fish that was fighting to survive, and it's a horrible feeling. That was maybe two years ago before I knew how to set up a proper QT tank, and I would never do it again. I am still stricken with guilt over it. Crazy, right?