When I use the keywords "free XML editor" in Google, I see several.
Joined: Jul 04, 2005
For non-commercial use, try www.freexmleditor.com - free, comprehensive, multi-functional XML Editor from Cladonia
Joined: Jan 04, 2006
I am developing an XML tree editor, called XMLDonkey. The version which is available on my website is already capable of displaying and editing well-formed xml. You can assign a schema or DTD to the xml to do real-time validition. There is still a lot of work to do; but maybe this version already is enough for your requirements. You can take a look at http://www.donkeydevelopment.com