The Lion, the one primarch who grew up hunting Chaos beasts. Yeah, I can see why Horus wouldn't want to deal with that.

I wouldn't wanna go against the lion if he won a fight against a literal psychic who can see the future

FOR THE LION!

It makes sense. The Lion was a good strategist, a great warrior, cold, ruthless, uncompromising, utterly loyal to his father, made the other primarchs feel uneasy in his presence, AND his legion had special access to the spiciest DAoT wargear that the other legions simply weren't trusted with. If I was going to launch a rebellion, the Lion is the smart choice to send very far away, because of what you do know about him, and perhaps more importantly, what you don't.

In family survival, you’re better off pissing of your dad than you older brother

I will forever love when the Dark angels were red silver and black and just stood on absolute business.

That Eldest Brother Energy, through and through.

The lion had access to dark age weapons that others didn't, right?

At the end of the day, there was just too much about the Lion that Horus did not know about and this is what gave the Warmaster pause. He was aware that there was an understated , yet very real relationship between the Lion and their father. He knew that the DAs were once commanded by the Emperor Himself that the legion retained many unique gifts, be it remarkable weapons, technologies and even biological enhancements for some of its longest-serving legionaries such as inherent psychic resistance.  At the end of the day, liars figure, figures don't lie - Once Lion, who was discovered much later than Horus, took over control of the legion, their compliance record surged in short order to #2, and that's just based on the formal battles/compliances - the ones on the books. The DAs were often off doing all sorts of nasty business on humanity's behalf in the darkest corners of the galaxy that nobody even knows about. It is noticeable in the books that Horus isn't just dismissive of Lion when he speaks of him... he's often rather catty. Referring to Lion as being unfit to lead since he was raised as a feral savage and that he could never join his new order since he's too stubborn and war-like with nothing else to offer. In fact, the only time he was ever nice was when Lion boarded the Vengeful Spirit unannounced and crashed Horus's victory party, calling the Warmaster out for daring to use his legionaries without even notifying him right to his face. Horus had conceded the point and give his brother kudos in front of everyone for managing to take him by surprise, despite no doubt seething inside.

Loyalty is its own reward.

The Lion can make you take a piss from across the galaxy by just calling out your name.

The Lion has Big Brother vibes.
They knew not to mess with him

Sorry, Rho, but gonna have to draw the line at Horus being a Physical match for Sanguinius. The trap at Signus was specifically to remove the emotional trauma and outright danger of an attempt on Sanguinius and the Blood Angels directly, and since the 9th and the 16th were so close, this was more of a calculated move to prevent anyone getting cold feet as their close friends were exterminated.
There is little doubt that in a 1v1 without being bloated by Chaos that Sanguinius would have had Horus’s head on his spear.
If Russ, could nearly kill Horus and he was saved only by his chaos powers, then Sanguinius would likely kill him.
The Lion is dangerous for all manner of reasons, but the most prominent is that he of all his brothers is the greatest hunter. Russ fights like a wolf and uses number and tactics to his advantage, but the Lion is a lone hunter and uniquely suited for it based on the lore.
Horus knew that while his other brothers would be quick to establish battle lines and fight straight forward the Lion is the one who would hunt him personally high and low and never stop pursuing him.
I mean if the Lion was capable of hunting down Curze and all but killing him and only because of Sanguinius was unable to finish the job, imagine that single minded focus on tearing the guys out of the 16th Legion and Horus specifically?!!!
No question that having Guilliman at the Seige would have made it nearly impossible to even take place, but it would’ve been interesting to see what the Lion could’ve accomplished if he’d been able to mount offensives.

Either way, fantastic video brother, but let’s not get ahead of ourselves in saying that a non-chaos bloated Lupercal could take out or even match Sanguinius

The Lion seems like the kind who already killed you 100 times in his mind

Big E has just sat there telling himself he' s an idiot for not giving the job to Lionel.

For all the reasons you gave. The Lion was, as a warrior, one of the very top strongest of all his primarch brothers. His strategical acumen rivaled Horus, some suggested even exceeded it? His legion was the first, they gifted all the other legions their known methods of warfare, which meant they could also fight well in all the manners of warfare the other legions were specialized in ... I also believe Horus had an idea that Jonson and the First legion were tasked with destroyed the mechanicum and Mars (being the Emperor's exterminators), and had weapons to do so that the other legions did not possess; and the Lion WOULD use them on him and the traitor legions in a war!

dont mess with the First

Leman was an honorable king pretending to be beast. The lion is a beast, pretending to be an honorable king.

Peter Turbo:  I managed to fool the Lion.  Did Horus ever fool him?  Nope!
😂

The Emperor sends Russ to kill his enemies, He sends the Lion to erase them from the map.