Except.... The Arbiter alone decide where a soul goes, not the soul itself, not Zovaal.. Only the Arbiter.
Thus, if the Arbiter was working when we killed Helya, she would have had to spend a lengthy time in Revendreth, in an attempt to atone her, before being sent on to the Maw (when failed to atone). That time haven't passed; just look at Kael'thas, he's still in Revendreth and he died YEARS before Helya, while having less to atone for.