Bananas can kill you if you eat too many of them and get a potassium overdose.
However to be able to get a potassium overdose from eating bananas you would have to eat 400 bananas in one day to get the amount of potassium buildup in your body to overdose on potassium and kill you.
So unless you have a very huge stomach it would be impossible for any human to eat enough bananas to cause a potassium overdose.
It's never dangerous to eat as many bananas as you want too in a day and they are actually healthy for you and you would not be able to eat enough bananas to be dangerous to kill you.
I've eaten 6 bananas in one day before as a good snack and never once did I get any bad health issues as a result and the bananas certainly didn't kill me as I'm still alive and typing this answer.
The bananas improved my energy levels throughout the day and I eat at least one banana everyday as a good healthy snack.