Pretty much any depiction of a Nurglite ever, it's actually extraordinarily odd for them to be somber or depressed.

Similarly, going off of stuff like Fulgrim and The Reflection Crack'd a lot of the changes the Emperors Children undergo is when they start questioning the Imperium and the restraints they put upon themselves. Their hatred of these restrictions inevitably tips them over to Slaanesh and makes Fulgrim go from regretting Ferrus Manus death, to embracing it.

Tome of Decay, Pg. 7-8
" All Chaos Gods have a dual nature, but Nurgle, more so
than any of the other Ruinous Powers, understands that the
supposedly separate elements of his essence actually work
together in a self-sustaining cycle rather than standing apart
from one another as different explanations of the same thing.
Khorne, for instance, is a god of bloodshed and killing—of
utter carnage—and also one of martial pride and a sense of
accomplishment or betterment. These two halves can be seen
as two sides of the same coin, but the coin must be flipped to
view and appreciate its obverse. But this coin is illusory; there
is no divide between its two faces, no beginning and no end.
The coin is nought but a feeble mortal metaphor for the truth
of Nurgle’s influence. On one “side” there is decay, death, and disease.
What would be on the other side of this coin is in fact part and parcel of the first side.
Hope, rebirth, resistance, and growth all arise directly from facing death and decay.
The Seers of the Eldar Craftworlds and the Inquisitors of the
Imperium will never share this truth with the weak-minded
fools who drink in their lies like mother’s milk."

Same book, Pg. 13:
" Nurgle’s Daemons and mortals alike smile at the
thought of serving their god. Other masters are harsh
and demanding, but the Lord of Flies only asks that
his servants embrace each moment. Though they are rotting,
diseased, and corrupted beyond redemption, Nurgle’s blessed
minions are rewarded with a sense of peace and certitude
about their ultimate purpose in the universe."

We're on the same page this part.

This is where I see differently. The Emperor like any nations leader trusts those in charge of military forces to uphold and further their nations ideals.
The Night Haunter though was just slaughtering everyone and their dog for untold number of planets that were wiped out by them.

His brothers complained to the Emps who did nothing at the time. The impression i've been given is that he didn't want to confront his son as he knew it would just make things worse. So things went on for a bit more than his bro's decided enough's enough and put his azz in jail until they could figure out what to do with him. But then he killed all of the guards and fled with his Chapter to the Warp. So even if the Emps doesn't see him as a failure, he definitely was a failure to the Empire.

Angron is in the same boat, his wanton destruction without much cause caught up to him, crossing the line of what could be tolerated.

They were two of the most blood thirsty Chapters of the Imperium and hence why no surprise to me that they were ripe for Chaos's picking.

They were however, also two of the most effective Chaos Legions, planets that the Night Lords wanted to conquer surrendered to Imperial compliance before any battle even happened. To me he approved of them, but his approval was silent. As one interpretation goes(And an equally valid one as the other, as stated by the authors.) is that the Emperor wanted Generals and weapons, not sons.

Which is something we could actually argue forever about, as it's left ambiguous on purpose.

Angron was treated as less than trash by the Emperor. Angron saw him for what he was from the moment he got picked up off of De'shea. A tyrant and a slaver. With Angron might makes right, Big E was stronger so he was in the right until Angron could find a way around that. This mentality also explains the bloodshed. His legion was mightier than the enemy and Angron had a killing impulse that he shared with his sons, so they killed because they could and thus they were right.

Thats does show he cares somewhat about his bro's but it doesn't do anything to show us an example of their minions performing such acts of good.

And if all nurgle daemons were like this, they would literally only ever spread disease and never go looking for a fight or treat others in a different manor than described there. But we know they do. Now i need to find an example.

They...kinda don't though, Plague Hounds...for example...don't actually fight...they tackle people to kiss them.

The resentment of everything they love dying is what turns them into rot flies, but that takes a long...long time.

Edit: Which is a broader point, Daemons don't see things the way you and I do, some of them might not even comprehend how fragile the human body is. When a daemonette stabs someone, and they stab back, it is essentially affectionate because they are trading sensations with one another.

Completely agree with you. As soon as I read how they put the spike in his head I knew he was going to have a tragic and sad life. Just like Night Haunter and his visions.

It was terrible how the Emps yoinked him out and just let his forces get slaughtered. In this I blame the author for just wanting to make it dramatic, because the Emps could have simply just sent down an allied force to help them win easily enough and/or bring his small force back with him to turn them into his Legions recruits. They didn't have the 1000 marine limit at that time. The Emps being as genius as he is would have just made sense to do it like that.

But the spike should have told the Emps he wasn't a good choice for leadership, but that opens a can of worms with all his other sons if he was to put someone else in command as well the gene-seed thing.

Actually, the resentment of the love that isnt returned from the receiving end is the start.
The actual catalyst is when someone manages to kill them while they are trying to kiss them, where the Beast of Nurgle is shocked to the core and perplexed at what it did wrong (cause they are like stupid puppies).

As it falls back into the pits of the mires in the Plague Gardens it teems with confusion and resentment for the way it was treated until it cocoons, lulled into sleep my thousands of droning flies.
Then after awhile the pod pupates into a full fledged Rotfly, a shadow of the loving demon it once was.

It is said that if they are lucky to meet the individual who "broke their heart" an banished them that they have a special fate for them. And that is being swallowed alive and being digested in its belly for centuries.

But yeah, Beast of Nurgle themselves are loveable creatures, who dont mean no harm, they are just too stupid to know better.

Fair enough! Slaanesh is my area of expertise, so I may get some of these things wrong when explaining the other gods behaviors.