He was probably overwhelmed by the infection, guppys, just like other small fish cannot fight off massive infections very well... they eventually just die from organ failure. Keep an eye on the other fish for any signs of infection for the next few weeks, they will most likely be fine though, infections usually only attack stressed or injured fish.
Edit : I would check all your water parameters..
ammonia,
nitrite,
nitrate, and
pH. Since you treated the tank with meds, you most likely killed your good bacteria and will be in a mini cycle. This added stress can cause the other fish to be more likely to get the same infection.