I know where DragoZero is coming from. Most martyr-ends I've seen in anime have been good, if not excellent.
OTOH, martyr-ends have become increasingly common in anime lately, particularly for big budget productions (a couple prominent 2011 anime shows made use of the martyr end, and that's just what I myself am aware of). There's actually a chance that much of the audience might go "ho hum, where have I seen
this before?" if GC goes with the martyr end so soon after two different prominent anime shows went for it. There's also the idea that Shu arguably deserves a happy end more than anybody else in this show does.
But I would encourage everybody to at least prepare themselves for a GC martyr end. GC has set Shu up almost perfectly to be a Christ-like figure if they want to go that way.
1) Shu's Void ability includes the ability to absorb the weaknesses of others unto himself, which also means he can heal people now.
Very Christ-like, just replace "weaknesses" with "sins".
2) "Lost Christmas" and Gai's cross necklace - This I would typically not attach much weight to in an anime show, but given the other hints out there, these really do seem to reinforce/foreshadow the idea of Shu as a Christ-like figure.
3) The very name "Guilty Crown" may make one think of the Crown of Thorns wore by Christ. This I wouldn't typically attach much weight to, if not for the fact that the song for GC's first OP even references "crown of thorns", IIRC.
You combine all of the above together, and it does paint a pretty clear picture, imo.