Email or eBay message?
With an email, you can Block the sender and/or report him to his email provider. Death threats are usually disliked by providers.
You should also delete the email. You are never required to open or respond to any email.
With an eBay message, again, never required to open or respond to emails. And definitely Report it to eBay before deleting it.
Also add this idiot to your Blocked Bidder List, even if you do not at this time do any selling.
With only 90% feedback rating, this seller is unlikely to last long on eBay.
I notice that no one (including you?) left Detailed Seller Ratings. He got an automatic 5 Stars for free shipping.
Detailed Seller Ratings are much more important than feedback for sellers. FB is an opinion but the anonymous DSRs are used by eBay to calculate how a seller's customer service is going. A normal transaction should get 5 stars. The lowest rating is ONE Star. No stars is a free pass.
But you are not innocent here. You got your refund for late delivery, but once the parcel did arrive, ethically you should have refunded that refund. You can't keep both the product and the money. Bad karma.
