Nathaniel Garro, the loyalist Death Guard shooting down the fighters pursuing Saul Tarvitz, the last loyal Emperor's Children Space Marine, willing to risk death to protect him because of his love and faith in his sworn brother of another legion.
Sindermann defiantly broadcasting a sermon of Peace and Tolerance through the War Fleet of The 63rd Expedition, pleading for the Love and Kindness left in humanity, stubbornly defying Horus' megalomania, not to mention the seemingly-hopeless Grimdark of the 40K universe.
The arguments between Garro and Varren in Legion of One and how Cerberus is convinced to stand down, despite being driven utterly insane by months of fighting and the events of Isstvan III. "The Emperor Protects."
The Outcast Dead features possibly the nicest depiction of the Emperor seen thus far, although that must be taken relatively. He only interacts with Kai Zulane, a thoroughly traumatised and frightened astropath, and tries to comfort him. He also makes a first in admitting weakness in that he can't know everything or do anything he likes. For the first time ever, the Emperor seemed less like a distant amoral psychic god, and more like a human being capable of relating to others.
In Know No Fear, the anonymous narrator pauses for a moment to explain the other meaning of the "Mark" of Calth.
Secondly, the "mark" of Calth refers to the solar radiation burns suffered by many of the combatants, principally the human (specifically non-transhuman) troops. The last of these veterans to die, many years later, still refuse graft repair and wear the mark proudly.
The final conversation of Ultramarine Ventanus and Skitarii Arook in Know No Fear. Two warriors from two very different cultures, who had just won a desperate battle against the treacherous foe. Two men who are about to part ways, knowing they will never see each other again.
Venantus: We march for Macragge.
Arook: We stand for Mars. *pause* It means the same thing.
Kharn, suprisingly, demonstrates a true brotherhood with Argel Tal. Argel Tal even describes him thusly: "He is the last brother I have yet to fail and has yet to fail me." In battle, Argel Tal continuously guards Kharn from death, deflecting blades, bolts and spears with no regard for himself. Considering what happens to Argel Tal, it is no wonder Kharn allowed himself to be lost to Khorne's will and the Nails.
Lorgar also demonstrates surprising compassion in the story with Angron, seeming to be the only Primarch interested in saving him from the Butcher's Nails. While he is vocal in his criticism of Angron and his Legion, he tries to find a way to spare him pain. He did, in a way.
And connected to this, when Cyrene is resurrected by Erebus, Argel Tal's first response to her return to life is: "You can see?"Important, because Cyrene was blinded in the past, and it says a lot that Argel Tal would notice it"
Four words from The Unremembered Empire - "We march for Macragge!"
Made heartwarming by the fact it is not said by the Ultramarines. It is instead said in unison by legionnaires from the White Scars, Iron Hands, Raven Guard, Space Wolves, Salamanders, and Imperial Fists who were coming to Guilliman's aid. And they say it to a frail old woman who is the equivalent of Guilliman's mother.
Another bit from The Unremembered Empire that takes some explaining: When Konrad Curze went on a rampage in the Fortress of Hera, it was believed that he killed Roboute Guilliman and Lion El'Jonson, but they were saved when Alexis Polux and Barabas Dantioch, using the Pharos, a piece of xeno-tech implied to be a precursor to the Eldar webway, pulls them all the way to Sotha, because of the device's empathic resonance felt the two loyalists need to save the Primarchs. When Sotha reestablishes real time contact with Macragge, Lion El'Jonson has trouble passing back to Macragge. Tarsa Euten then informs Guilliman that Curze nearly killed her, and walks through the field without even realizing it. How? He needed to make sure his mother was okay.
Let's not forget Dantioch and Polux, who become fast friends, despite being an Iron Warrior and an Imperial Fist. In fact, the ability of Pharos to transport people gets discovered because wanted to get Polux to safety so badly.
The Wolf of Ash and Fire
‘I was there,’ he would say, right up until the day he died, after which he spoke only infrequently. ‘I was there the day Horus saved the Emperor.
Really the entire story is a Heartwarming Moment for Horus and the Emperor, the father-son relationship that all the sources talked about is finally shown. Not to mention Horus leaping into a chasm without any hesitation to follow the Emperor as he fell. Which is also why the entire story is also a Tear Jerker, because we are finally seeing exactly what will soon be poisoned and destroyed by Chaos.
The sheer amount of restraint that the White Scars exercise when the conflict between those who favor Horus and those who favor Terra inevitably comes to a head. No active purge of its members via surprise betrayal, like the Luna Wolves, Emperor's Children, Death Guard, or World Eaters. No for the Evulz dog-kicking like the other traitor legions at Isstvan. Just a sudden surprise takeover that would have succeeded relatively bloodlessly if it weren't for a meddling human and an observant loyalist Khan, who in turn tries to avoid killing his fellow comrades if at all possible even as he's fighting his way to the legion flagship. Not to mention the only reason Chaos ever came close to subverting the legion was by manipulating their natural trust for those they consider Blood Brothers, as well as how it is ultimately resolved not by a bloodbath but by Jaghatai Khan and Targutai Yesugei arriving in time to reveal the truth that they spent half the book uncovering. Turns out Ignace Karkasy was right, back in Horus Rising: the truth is all we have.
The Perpetual Alivia Sureka and her adopted little girls Miska and Vivyen. Her relationship with them is a Heartwarming Moment as she uses her psyker powers to make sure they escape Molech, and when she survives to join them. The book ends with all three of them reading The Little Mermaid. Made this troper smile.
Fulgrim, seemingly genuinely, complimenting Perturabo's vision of a rebuilt Olympia towards the end of Angel Exterminatus. Given that by this stage Fulgrim is not only a colossal bastard, but has spent most of the book trying to use Perturabo's life energy to ascend to daemonhood, it's surprisingly touching.
The audio-drama Honour To The Dead has a squad of Ultramarines - a Legion often characterised as being prideful and arrogant - in the middle of the battle of Calth - the worst disaster to befall their Legion - helping a random woman and her baby get to safety, simply because it's the right thing to do.