Fen gives no Rivalry for letting Castillon go and no friendship for killing him.
Aveline, of all people, will give Rivalry if you kill him and friendship if you let him go, but that's par for the course with her, she's at her most hypocritical when it comes to her friendship with Izzy.
I've never seen much point in not killing him, though. If you're Friending Izzy she doesn't really care, and if you're Rivaling her you're actually doing her a favor because she'll feel really bad about it later if you don't. To agree to her deal your Hawke has to a) not hate slavers, a2) not feel a general responsibility to Castillon's victims, past and potential,

not be at all worried that Castillon will break his word and remain a threat, c) not be irritated enough at Izzy's mild double-cross to want to mildly double-cross her back, d) not be governed by any secret shameful lust-related ulterior motives to want her to stay in Kirkwall, e) not be dedicated to
sanctimonious prathood teaching her the error of her ways (I kid, I kid, I don't hate the Rival path).
Thus far all but one of my Hawkes have fallen into at least one of those categories. So poor Izzy tends to stay landlocked in my games.