"Every little death helps" That is a whisper that actually might back this idea that Helya's corruption/rebellion might not have been as evil as we may have believed.
The Void Lords need souls to power their crossing into our world. If you think of it Undeath kinda prevents that from happening. Instead of spiraling into the void they just can't die "officially." That is they cannot power the void. To allow a true Void Lord into Azeroth you need to toss a huge amount of soul power into the mix. Helya wasn't serving her purpose ... on purpose then. Odyn can no longer traffic souls which he had no right or need to do in the first place. Who really is known for needing/devouring souls ... Old Gods.
So why would Odyn see that as a bad thing? (Puts tin foil hat on.) Because Odyn's true purpose was to gather only those he considered worthy of serving under his Divine Order and feeding the rest to the void like a trash disposal. Helya knows that once she's gone all of these unlucky unworthy souls won't make it back to the Halls of Valor will go straight to N'Zoth's belly makes a deal with Kerrigan.. I mean Sylvannas that should she fall she must guide the damned away from oblivion before Odyn has his way.
Meanwhile N'Zoth having his servants Azshara, Xavius, Cragshaper/Dargrul*, and ODYN (whispered) practically hand the 5 pillars to us. He simply waits for Gul'dan's inevitable fall after Nighthold to bring them all together. Then all of the souls Odyn has gathered can be poured to the void and the way to N'yhthola can be opened.
You may ask why all the Odyn hate? Well it has to do with the fatal response to "Odyn was a great enemy... until his own arrogance proved his undoing. You should ask him if he's considered Loken's final words to him. Go ahead. " Considering that Loken who later fell victim to Yogg Saron's whispers also released and suggested to Helya banishing Odyn to the Halls of Valor all it may have took for Loken to lose faith in Odyn or visa versa was a simple question.
"Have you heard the whispers?"
*Xal'atath whispers: Though he is not one of our servants, he has heard our whispers. They darkened him forever. (at Ularogg Cragshaper)
*Xal'atath whispers: A beast of stone who ignorantly throws around power he barely controls. He should be put down before he ruins our plans. (at Dargrul)