Eating too many bananas in a day could kill you due to Potassium Overdose but to get the overdose of Potassium you'd need to eat 274 bananas per day in order for it to be a problem and kill you from potassium overdose and radiation poisoning.
You'd likely get a stomach ache before you consumed 274 bananas so eat as many bananas as you want to in a day or can consume and you'll be safe.