It is easy to cast some runes as benevolent spirits of light and love. Thurisaz is not one of those runes. The Thorn has the personality of a rabid pit bull with a toothache. It will gladly tear into anything you send it after, and if you aren't careful it will turn around and chew you up as well.
You can read the rest of Kenaz Filan's useful essay on Thurisaz over on his blog.