Follow this link and read the actual document (it is only 10 pages, easy reading). Yes, it is through the HuffingtonPost, but all other sources on the story in the first page of the search are news stories and opinions withou tthe actual unclassified document. So, for lack of a more acceptable source for some of you, the Actual DHS Report will have to do.
No where in that report does it say that veterans will become terrorists. It does say however, that under the present circumstances in the US, they could be targeted for recruitment by right wing extremist groups.
And if you are interested, here it a link to the Left Wing Extremist report from the DHS. Through a link on Fox News.