Help! My betta fish just died!!?

He had fin rot and I gave him tetracycline but after 5 days of no improvement, i discontinued. I was going to give him kanacyn so mpletely cleaned his 1 gallon tank. After putting the water conditioner in, i waited 30 minutes. Then I took him in his little cup and put him back in his tank. As soon as i put him in, he started darting around ballisticly and jumping and banging against the tank glass and gravel. Then he died! :( How did he die?? I am 100% sure I put the water conditioner in and the aquarium salt. What happened to him?? He was fine in the little cup! I've had him for almost a year and he has had fin rot for a long time. I didnt even give him the kanacyn because he died as soon as I put him in his tank. :( PLEASE HELP! 5 star rating for best answer. I loved this fish! :(