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I've been watching Legend of Korra and I feel like there are some unanswered questions about the metallic poison that Korra was administered at the end of Season 3. While Zaheer makes it clear that the poison will draw out the Avatar State to help protect Korra, this seems to contradict everything we know about the Avatar State up to this point. It appears to imply that the Avatar State will automatically manifest itself to protect the Avatar if the Avatar is in danger. However, we know from previous episodes that:

• A fully realized Avatar has full control over entering the Avatar State.


• Korra is a fully realized Avatar, having shown the ability to control the Avatar State and connect to the Avatar's Cosmic Energy. Compare that to Aang, who spent almost the entire 3 seasons of The Last Airbender being unable to directly control the Avatar State.


• Avatar Roku was killed by a volcano, and he knew his death was imminent, but the Avatar State did not manifest uncontrollably to protect him.


• If the Avatar State always asserted itself like this, then the Avatar Cycle would end once an Avatar is killed through unnatural means (combat, poison, natural disasters, etc) which seems like a pretty terrible defense mechanism.


• Both Korra and Raava, who together define the Avatar, know that dying while in the Avatar State ends the Avatar Cycle and would presumably be inclined to preserve the Avatar Cycle over preserving Korra's life.

Considering what we know above, how is it possible for the poison to threaten Korra and draw out the Avatar State in an uncontrollable manner?

Going into the Avatar state doesn't heal you, it just protects you from danger. Roku was fighting a losing battle and so going into the Avatar state wouldn't have healed Roku's lungs or Wan's injuries.

The difference between these scenarios was that Korra's body was healthy but her life was directly threatened. My guess is since the poison is metallic the Avatar state can help remove it and then deal with the other threats i.e Zaheer and his gang. Unlike Roku who can't remove toxic compounds from his blood or Wan who can't well, heal. That's just my guess but I mean this is the Red Lotus we're talking about, I don't think they would have done it if wasn't going to work.

The Avatar state is still a defense mechanism that will protect the Avatar in the event that they cannot protect themselves I.e loss of consciousness, being bound/chained, blood bendered, drugged, seizures, etc

I was going to add this as a comment to @BhlackBishop answer, but as I
was writing it, I expanded on the subject much more than I was
expecting, so I decided to add what I think to be a complementary
answer.

We've seen at least three different contexts where the Avatar State is activated in the show:

• Consciously — where the Avatar enters the Avatar State by its own free will;


• Emotional distress — where the Avatar enters the Avatar State when he/she is put under some sort of emotional distress. An example being when Appa is captured by one of the Sandbender Tribes, Aang loses his temper and becomes infuriated, entering the Avatar State and only leaving it when Katara calms him down;


• When his/her life is threatened — the Avatar enters this state not by choice, but because his/her life is under a imminent threat.

As @BhlackBishop already mentioned, when the Avatar life is threatened in a manner that can be counteracted, the State might be automatically activated in order to save the Avatar.

We could argue that the Avatar State is intended to be used only on extreme situations.

Otherwise, why the Avatar State isn't activated, consciously or unconsciously, whenever the Avater enters in a combat situation?

From a pragmatic point of view, even if slim, there is a chance for the Avatar to be killed on every fight he/she engages.

With the sole exception being when an untrained and inexperienced Avatar can't get a hold of himself, the Avatar State is a contingency for situations where the regular Avatar abilities and powers aren't enough to deal with the problem in hand.

When Korra was poisoned, we could see that she was entering in the Avatar State unwillingly, but she was struggling to leave the State as quick as possible. In the first moments, she wasn't staying in the Avatar State for more than a second. This was happening because Korra was conscious and aware that if she stayed in the State for too long, she could be killed and the Avatar Cycle would end.

Even though there's the automatic Avatar State activation when the Avatar life is under imminent threat, the Avatar can still control, with obvious limitations, whether this should happen or not.

The Avatar State being activated while Korra was poisoned is actually a great defense mechanism, it's what gives her a window to escape and almost defeat the Red Lotus members. We can't know, but maybe, if Zaheer and his gang were defeated in that moment, Korra could leave the Avatar State to die by poisoning and not end the cycle.

The context of Korra's poisoning was really particular, keep that in mind.

The Avatar State as a defense mechanism, and as most defense mechanisms we find in nature, has its inherent disadvantages, doesn't cover 100% of every possible scenario, but so far, it has been effective, after all, the Avatar Cycle didn't end.

About Roku, remember that in the volcano scene, his dragon lands near him and surrounds him with its own body, when the dragon did this, Roku was still alive. Thus, we're unable to see in detail what really happened in Roku's final moments.