After Kahn revealed his intentions, there was no other choice left for Sirius but to accept his new employer's commands and sign a blood-bind contract with him on the spot.
Kahn then revealed his plans about what to do when they entered the arena and his own matches started.
One of the main reasons Kahn decided to participate was also to earn billions through the competition. Because even now, their forces and influence such as Bloodborne company or the Seven Deadly Sins underworld organization were powerful only inside one district.
And even there, they were lacking manpower and funds in case a full-scale war broke out in the future with powerful opposition or some noble clan.
Raising and feeding an army of their own wouldn't happen overnight and the amount of money as well as resources they'd have to spend would be manyfold greater than they currently had.
