What to do with a deceased owner's used iMac?

Without more info the following are “educated guesses.”

Assumptions:

• The computer is protected by a password which is unknown to you or to the estate.
• The computer will go into “activation lock” if a reset is performed.

Your options are:

• Have the decedent’s estate executor perform these tasks: How to request access to a deceased family member’s Apple Account - Apple Support after which you will be able to reset and use the machine as yours.
• Turn over the machine to an electronics recycler

Personally, I would have it recycled.

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