Mattimus2

from what i have gathered, if you remove the barrier before you speak to the moth priest, he will become friendly when you defeat him and speak to him so all will be well. if you speak to the moth priest before you remove the magic barrier he becomes friendly. simply do NOT remove the barrier and all will be well. removing the barrier seems to trigger his dialogue about how you must pay for killing his "master" and now he's hostile. if it is too late to avoid this and you can't go back to a prior save, you can use console commands to complete the quest instead so at least you can progress. i have used both ways, obviously the first option is better because then "mothman" will chill at dawnguard and you can have dialogue with him... good luck, this is a frustrating glitch.