"The original site changed to a different domain name." (users can find it by now surely)
"domain spent the last couple of years as a parked page"
(who misses it now?)
You won an auction, other bidders would keep it.
Lastly, this is why you don't let important domains expire.
If you buy a domain off the drop, the owner has been contacted multiple times already about it. Presumably they realized when they saw the parking page when a domain goes into pending delete mode, that the domain had expired. For whatever reason, they didn't renew it.
Still, do what you feel is right so that you can sleep soundly at night - that's the most important thing. I think morally you're in the clear, but if you don't feel right about it then do what will make you feel better.
Let us know how it works out.
It's always worth listening to your inner Good Samaritan, but don't be too quick to make a gift of the domain unless you're confident that they'd handle it more competently than last time.
The middle ground might be to link to the original site's new location so that users who followed old links could still find it.
If the domain was allowed to expire not once but twice, there's a good chance that Page Rank from the existing links won't be counted. Enjoy whatever traffic you get from the links, but other than that, expect that you'll have to start from scratch to build your own web presence for the domain.