Moiraine even says to Siuan, "If you have ever loved me, you won't make me do this." And Siuan does it anyway. Additionally, Moiraine warns Siuan not to attempt to take one of the Forsaken. And Siuan does it anyway. From my perspective, Siuan proved unworthy of the love and, in doing so, lost...
Meh. Given the fate of the world and the choice between leaving Rand with a member of the Forsaken and helping the person who had just betrayed all the trust and love you'd built... for me, it's not a hard choice.
As for not leaving anyone like that... meh.
I mean, Siuan had also just compelled Moiraine to obey her with the fealty oath. If it were me, I'd think, yeah, you're good on your own.*
*That is the kind, expletive-free version.