Well maybe because it was endangering them all, wasn't it? hadn't Irina and the rest of them drank the water? (i haven't watched the 5th Season, dunno whether Irina makes a reappearance)
However, i possibly think Elena Derevko......she's probably the only one who wanted to kill everyone...
Hey, my name's Thomas, 14 yrs old, male from Australia. Extremely avid Alias fan, only watched the first episode of Season 3 and got addicted! Such a mad show! Hope to talk to all of ya at some stage!