Interestingly, Chandler practices Divination magic. Magic that Jim has said skates the borders of the 4th Law of Magic (Thou Shall Not Swim Against The Currents Of Time - or whatever it is called). Which is why he is normally always near the Senior Council. Basically, he mostly is an early warning system.
So, with all that in mind why didn't he see this coming? Or did he see it and react badly? Or did he see it and react as planned?
Personally, I think he isn't a traitor as such (for now). At first I read it as Drakul killed him, and I think that's what Jim was trying to get us to believe. But upon re-reading the section and hearing some theories I agree that it's more likely he was sent *somewhere* for Drakul's use later. I don't think it was a handy exit for Chandler for several reasons:
1) Why disappear then? Plenty of chances before that and to do it easier
2) If Jim really wanted to mislead us about his loyalty, probably would have been smarter to make it a more sad/heroic death scene.
3) Drakul seemed irritated by Chandler's attack, and I think Jim was more likely showing just how easily Drakul can deal with things even when mildly annoyed.
4) Drakul's powers, and the goals that LtW and River seem to suggest he has, would seem to be served better by turning Chandler.
Also, Jim payed special attention to the shape and nature of the portal. Not just any old Way. Obviously, he could have sent him anywhere but I think it was laying groundwork for something bigger later.