I have a gut feeling it's deeper than that though...Ella isn't one to make stupid decisions of such a grand scale as S.I.N and I'd assume Langdon is the same way given his status in the world.
They both HAVE to know how utterly powerful The Lvl 5s are, and yet they insist on rivaling them...
My take is that all that confidence isn't for show, they have SOMETHING under their sleeve...
I don't think it's a Monster, that includes Apsotles; as even the ones Ella prepared at the Battle of Diamonds was just to at best "distract" the higher Lvls, and the 3rd & 4th only interfer with Monster affairs from what we've seen...so then what is it, any thoughts people?
Some considerations:
1) There is one indication Langdon doesn't really know what is going on with HERO. That's the SM seemingly avoiding his calls. Langdon seems a bit surprised, even if he says that he wouldn't do that without a reason. I think SM knows about Langdon and doesn't want to deal with SIN.
I don't expect that Langdon is completely clueless, since he deals in information and can observe HERO operations with his satellites, drawing some conclusions. The thing is that the vast majority of monster threats are rank A and below, which are hardly even an afterthought for the captains. Langdon does not have the means to gauge the extend of their power. A newborn level 5 can deal with these monsters as easily as a captain can.
After all, Langdon knows very well what Alexis can do. That sword is enough to bring down his Skyscrapers and Alexis has improved significantly since then.
2) Langdon took advantage of Ella's plan to grow SIN. HERO were focused on dealing with the monster threat, so he had ample time to grow his organization. Now that Ella is dead and the monster attacks are less frequent, SIN is back in the crosshairs. It barely took a week for HERO to plan and execute a plan to unravel Langdon's operation. Now he is exposed as a superhuman so HERO can probably legally go after him, HERO have information from SIN's database and a plan to infiltrate SIN directly.
3) As long as the Hexenringe is alive, Ella doesn't need any help from stronger monsters. One important piece of info that a lot of people forget, is that Ella wanted the Diamond Battle to happen and she had already finished her business there. She could just go into hiding instead, but it was a good opportunity for MC to grow, her to test the waters with HERO, mark HERO agents etc.
Additionally Ella has help from Deus. Deus deceived Alexis. He told her that he is the only one who can corner the Hexenringe, but he probably took a nap in a cave near where a possessed MC would land if the bad scenario came to pass. He also probably deceived HERO leadership by ommiting details about the battle's result.
The Hexenringe was always free to spring it's trap once the conditions were right.