It looks like a shot wound.
Sequences:
1) Looks like Rolo was looking at the gun when Shirley pulled her finger away from the trigger when she thought he was an ally
2) Rolo's expression isn't positive when Shirley says she wants to protect Lelouch
3) Gasp and turns on Geass when Shirley mentions Nunnally.
4) Flash to the locket Lelouch gave him
5) Symbolic showing of his mask falling off (her not knowing he's an assassin reasoning fits here)
Doesn't look good for Rolo. If he blames Suzaku, that's a red flag. I don't see any logical reasons Suzaku would do it.