Oof, OOD Sub Zero Bi-Han is a weird fucker, and he's not too keen on letting his little brother out of his grip.
Copy pasting a comment I left in reply on this one because it kinda expands on what's going on here
"The letter and assassination attempt are real, Bi-Han's just keeping an eye on them to make sure they don’t get too close. He's spent a lot of time isolating Kuai Liang in the hopes outside interference won't make him sway from his side and this is essentially the first time he's allowing him to properly interact with someone else. There is a test aspect to the entire situation, both to see if he can lift his restrictions on Kuai a little without risk but also partially to test Hanzo's loyalty.
Bi-Han's hatred for Hanzo is partially the murder aspect (he's seen it happen multiple times thanks to the hourglass) but it is also because he sees Hanzo as one of the catalyst of Kuai Liang straying from his side. He is however also very aware that Hanzo is a powerful warrior and wasting that potential would be foolish of him. He's just gotta make sure he's on a short leash is all."