My Dad gave me his cards and pin numbers just before he died. He's left everything else a mess, no will, my dead mother's name still on his life insurance policy and 401k. Is it illegal for me to use these cards? He lived in the state of Florida.

Strictly speaking, it may be. Gather together all info and documents that you have and talk with a probate attorney immediately. (S)he will be able to advise you on how to proceed to straighten everything out.

Once a person is deceased only the appointed executor of their estate can legally access or control assets of the estate.

You will need to probate the estate to establish the heirs to get the life insurance and 401(k) funds.