News of the Weird reported the following in their 1/8/6 edition (.935):
Mr. Rayfran das Neves Sales was convicted in Belem, Brazil, in December of the widely reported murder of an American rain forest-activist nun, who was gunned down as she argued with Sales over who owned the land he was working. Sales claimed self-defense, in that, according to him, the nun reached into her bag as she was proclaiming that "the weapon I have (for fighting for preservation of the rain forest) is this," and Sales, sensing that she was about to pull a gun, shot her. The nun's "weapon," was, of course, her copy of what countless preachers refer to as a primary "weapon" against sin: the Bible. [Tampa Tribune-AP, 12-10-05]
Here is an article.
I suppose we don't know whether he is being truthful or not. On one hand it's not an entirely implausible story and it's something (I'd argue) that would be tough to manufacture. On the other hand it sounds like he was incredibly trigger-happy given the circumstances, and he was undoubtedly a notorious individual who had plenty of motive to kill her.
Very sad if true.