You're missing something important in the bolded part.
Originally Posted by Bonsay
Electrotaxis is movement in response to electric stimuli. That means the cell is not 'carried' by the current, the cell moves by itself towards the electrode.
That's why it stops when it dies- dead cells don't respond to stimuli and don't move by themselves.
I think you need to explain why it died, not why it stopped moving.
And no, I have no idea why it would die, but try looking for something that has to do with the membrane potential (voltage difference between cell interior and exterior). The cell needs that potential to survive and it needs it to sense changes the electric field.
If you do find an answer, please post it here. It's an interesting question.
"Hyperpolarization of the membrane potential causes an increase in the beating frequency of cilia and depolarization causes ciliary reversal." So, Paramecium needs to depolarize in order to change direction, but I don't know how this would kill it. Maybe because it can not repolarize in pure water?