There have been a couple of discussions on that here, I'll see if I can dig them up for you. From memory theories I've seen are that Andross had it done by someone to use him as a sacrifice for the Blinder's Knife, that the wight was someone out for revenge on the Guile's, that it knew about the prophecy Dazen-who-became-Gavin had hoped Sevastien would fulfill, and that Dazen-who-became-Gavin killed Sevastien himself upon drafting Blue for the first time and then blocked it out from the trauma or something, substituting a Blue wight for himself in his memory (I don't find that one very plausible).
A thread on this topic: http://brentweeks.ning.com/forum/topics/sevastien-spoilers
The one with the weird theory that it was actually Dazen that killed Sevastien: http://brentweeks.ning.com/forum/topics/gavin-rl-dazen-killed-his-y...
So my issue with the Sacrifice theory is that Sevastian was too young to show what colors he could draft, right? So I don't think he was draining the colors with the blinder's knife. However, I think there is some evidence for the sacrifice due to familial relations. I think a previous prism was a White Oak (known to have several kids) and also Aris (sp.) Green (the one who is serially pregnant). If becoming an artificial prism, through the blinder's knife, requires a familial sacrifice, that is further evidence.
Yeah, Sevastien was still just a kid, so I don't see how they could have known whether he could draft at all, let alone what colours.
I don't really think we've seen anything to suggest that becoming Prism requires familial sacrifice, but if so then yeah, that could make sense.