how about:

he wants him to use bf to further hurt him and the pattern/turn him toward the dark

semmy was a patsy. the torture made him darker than ever and the situation created an opportunity to force rand to channel the TP to BF semmy

also now the forsaken would know rand will use BF to kill them. i think this gives graaendal a heads up when she gets Rand's messenger.

with rand and grandael in arad domon, rand attacking graendal could be expected. Graendal avoids the attack and moreover makes Rand think she is dead for good.