With the "is[current]" bit, it looks like you're trying to make view template for the button. But since every single tiddler already has a delete button, I'm not sure what the point it.
As a view template, this works:
<$button message="tm-delete-tiddler" >Delete</$button>
</$list>
Note that the message is "tm-delete-tiddler" and you don't need fieldmangler.
If you're trying instead to make a list of items with the due field and their delete buttons, then something like this should work:
<$list filter="[has[due]]">
<$button message="tm-delete-tiddler" >Delete <<currentTiddler>></$button><br/>
</$list>
Good luck!
-- Mark