All the people who had gone to attack Emily were either of the First Order or lower.
And the reason why that mysterious person used those weak guards instead of sending someone of the 2nd Order or even stronger was that if Emily really had died then it would have been impossible to link the murder back to the mysterious person by only using these weaker people since there are a lot of them.
But if the ones who killed Emily were stronger and they got caught, they could be linked back to someone who they were working under, and that's why only these small fries were sent there.
The door was locked from the inside but the window was open so Kai went in through there, the man lying in front of him was in the 1st Order, killing him now would be extremely easy but Kai had another motive behind coming here.