Even if Pandora did mark Dex, the amulet used to boost his magic power left him "disoriented and open to the suggestions of whoever was able to get through the disorientation" (here). Maybe Sirleck/Volty/Lord Tedd/Phil/The Gods of Curling/etc. took advantage of that to trick him into attacking the Dojo. (Maybe that could be why Pandora later stopped using amulets to boost magic power, and blocked the magic flow instead.)
OTOH Pandora herself seems to think of the Greek myth being about a box, so probably wouldn't use a jar as her logo. So maybe that adds to the idea that someone else gave the amulet to Dex and she's being framed...