It strikes me that if you don't have any particular target in mind, it might help to consider whether/to what extent you want the character to have to hide that hatred within Riftwatch itself? For example, if he's out to get Venatori, his goals are pretty aligned with the organization, and it's going to be a matter of degree of force (not giving quarter to one who has actively surrendered, killing someone Riftwatch had planned to interrogate, etc.). There could be some cool stuff with hiding identity still, but it's not necessarily going to feel like A Betrayal if/when it comes out. On the other hand, if the group the character is angry at is well-represented in riftwatch (humans, mages, etc.), you've really got a different flavor of hiding a secret and a very different scenario for the secret getting out.

I think either approach could be fun, but they're so different that it's really the first place I'd want to narrow it down if I were in your shoes.