Apr 8th 2008 8:33PM Given the blizzard lore, it's not too much of a stretch to believe that Kaelthas is delivering his people to the Burning Legion. Given the preferential treatment other races have gotten (Eredar come to mind, but also Nathrezim and so on and so on) it is not unthinkable that he thought that would be a good idea.
Of course, Kil'jaeden was the Legion recruiter before becoming head honcho after Sargeras' untimely exit, so Kil'jaeden was probably more forthcoming than any of his previous masters. Add to that the corrupting influence of vast amounts of fel energies and you get a good mixture.
To be quite honest, Kael'thas choice is not even such a bad one given the circumstances.