"Why did Krishna, albeit indirectly, assist Duryodhana in annihilation of his own Narayani army?"
Some believe that Krishna wanted to reduce the burden on the earth, which included his own invincible army as well. War of Kurukeshetra (MahAbhArata) provided a great platform for that, even without him not being the participant. However, this can't be a reason given to lot of families who would loose so many fathers, husbands, sons in the war. Not necessarily, all can be burden on earth. In that way, Changez Khan, Hitler, Stalin, Mao, ... all will be justifiable after few centuries with this 'burden' reason. Were they all Krishna-like incarnations of Vishnu?
Due to its nature of speculation, it's better to leave it unjudged for now. May be the subsequent Q might answer it.
"Does it mean Krishna had indirectly taken part in Adharma?"
No, this was not Adharma.
"Then how was it DhArmik?"
This wasn't Dharma either! Due to number of lives involved, it was even more severe compared to what Krishna was later going to do in war. He didn't follow any of the following Dharmas:
- As a Warrior, he should have actively participated like his previous wars
- As a King, he should have thought first of his citizens and army; By
offering to Duryodhana, none of these gained anything but destruction
- As a Righteous man, he should have sided directly with Arjuna and
straight forwardly reject Duryodhana any help; Not only it would
shorten the war, but the amount of destruction could have been less
The Krishna had all of above titles, but he didn't follow the required Dharma for any of them.
So if it wasn't Dharma then why it was not Adharma either?
Let's try to understand all the related terms:
- Dharma: It's all about righteousness, naturality, bliss, knowledge & all. SAttva is Dharma.
BG 18.30 — The one who knows activity & retirement, duty & non-duty, fear & boldness and bonding & liberation properly, has SAtvika Buddhi.
- Adharma: Actions deluded with purpose (Sankalpa) to gain something or in awe of fruits. Rajas is Adharma.
BG 18.31 — The one who doesn't know Dharma & Adharma, duty & non-duty as it is, has RAjasi Buddhi.
- Vidharma: Acting without understanding the context, following something blindly. Ignorance triggers misinterpretation of Dharma. Tamas is Vidharma.
BG 18.32 — Covered by darkness(blindness), the one who believes Adharma as Dharma and believes other (thing)s also wrongly, has TAmasi Buddhi.
All the 3 gunas keep fluctuating in all the beings. Mostly it's true that, Sattva is the right way, because that follows Dharma. But it's not the ultimate truth, because, the less known fact is that, like Rajas/Tamas, Sattva / Dharma also binds the jeeva and keeps away from Moksha.
BG 14.5,6,7,9 — O might armed, the modes of Sattva, Rajas & Tamas generated from Prakruti, binds the embodied Indivisible (Atman) into (subtle) body. — O sinless, Due to purity, the Sattva is illuminating & distortionless. It binds with happiness & with knowledge. — O son of Kunti, know that Rajas is generated from self-desire & thirst (to achieve). That binds the embodied (jeeva) with Karma. — O Bharata, know Tamas to be born from ignorance. It binds all bewildered embodied (jeeva) with enjoyment, laziness, sleep.
A SAtvik or DhArmik person will be illuminated with knowledge & would be able to remain happy & content. However such heavenly feeling also binds (yet detached) the jeeva. Best examples are Devi-Devatas. Such beings are not limited by pUnya Karma. Following Dharma also brings a jeeva to higher/heavenly planets. The SAtvika beings usually worship Devatas (as BG 6.41, BG 17.4, but not to be confused with contradicting BG 4.12, 7.20) & also receive such higher planets when they die (BG 14.14).
BG 7.13 — These whole universe being bewildered by the nature of these 3 modes/gunas, doesn't know me who is above (them), the transcendental Indivisible (Atman).
Krishna contained these 3 modes (mainly Sattva), but was not bound by them. He was well aware of Dharma & Adharma, so surely he was not RAjasika/TAmasika. He didn't follow Dharma, so it was neither SAtvika.
Here comes the 4th category:
- Dharma TyAga: A more casual/famous term is Leela. "Dropping of Dharma created out of Prakruti". This can be also termed as assuming "ultimate Dharma".
BG 18.66 — Leaving away all the Dharmas, surrender unto Me. I will liberate you from all the sins. Do not repent.
Here "Me" is not the physical Krishna, but the supreme One (talking as of embodied). Whatever Krishna preached, he also practiced. There are many examples of 'not following Dharma'.
- Giving away Army to Duryodhana
- Not fighting for Arjuna
- Deceit while killing Drona
- Disguised entry to city for killing JarAsandha
- Running away from battlefield
- Instead of conducting session on "Women safety", Krishna took
shortcut by sliding away gopis' cloths onto tree!
- Instead of educating the villagers on "labor rights", Krishna chosen
to steal the butter pots!
"Seriousness/Importance" of events is our perception, but for enlightened beings (here Krishna), the treatment of any event is equal, be it funny or not.
This neither means that we have to abandon all our Dharmas nor it means to ape Krishna's activities. All the above bullets were SwaDharmas of Krishna, not ours.
"So when to drop Dharma?"
Everyone are different and our inner consciousness is the best judge of this. Important to note that, even to drop/leave away Dharma, the possession of Dharma is required. That's the reason, avatAras incarnate to uphold it within society (BG 4.8). Adharma or Vidharma are imaginary and can't be dropped.
When we are aware that whatever Karma are being done, we are NOT the do-er.
BG 3.18 — He/She has any purpose (Artha) left for neither acting nor inacting. And neither he/she has any purpose or dependency on other elementary beings (bhuta)
BG 3.27 — All Karmas are enacted in Prakruti by  modes/gunas. Bewildered with ego (false identity), the Atma (true self) believes "I am the do-er".
A rough analogy:
When a lovely kid playfully asks for a toy, we sometimes don't give away directly. We may distance it and let the kid go there, then we take in other hand and again distance it in opposite and let the kid go there. The toy belongs to kid, for which it's making sweet cries. But we don't give it so 'easily'. Finally we give it away with a smile.
- We know that, the kid wants to play with toy rather than it being in our hands, so not TAmasika
- We know that, toy belongs & to be given to kid (Dharma), so not RAjasika
- We don't straightforwardly give away the toy in dutiful way, so not
- We are just doing Leela