My gf won't allow me kill rats, is she insane?

How about a compromise, instead of killing them or relasing them, how about you just depress them a little? Put some rat poison near their cage, and if you can make them sad enough (i recomend having them watch the first half of ratatouille, where remy gets seperated from his family), they will choose death. You can't really say that is inhumane because you aren't killing the rats, they are killing themselves.