What do you think about me demanding to be paid for watching my friends now dead cat?
Do you think you should get paid?
I think that they would pay for it.
You could ask them politely but demanding maybe wait awhile, as they are probably in mourning. Some people just think of their pets as family. They should do the right thing either way and pay you for your time regardless.
They are in a better emotional place, they no longer have their cat.
Well you should still ask for the money, because you are correct that you should get paid when you do work. They're not being very kind towards you by not giving you payment.