Yeah but the second background check had nothing to do with the company. Tae wanted to know more about Siyeon personally. Now that the contract killer issue is known they could just talk openly to each other to get closer but yaoi. Anyway this is more fun for us.
As I said below Siyeon not doing a background check on him would have been foolish. He needs to know what type of guy his roommate was unless he wants to potentially be stuck in a bad situation yet AGAIN. That's how many of those scenarios happen in real life too. (I'm seriously not understanding how people are so naive in this day and age.) And similarly to what the other poster above mentioned Taekyung was doing a totally work unrelated and UNNECESSARY *second* background check.
Do you guys not remember Siyeon WORKS for Taekyung's company, and not as a normal employee but as a CONTRACT KILLER. Of course they had to do a background check on him, it would be too risky not to. I'm not justifying it, just stating facts. Also, Siyeon kinda did something similar (not to the same extent though) when he first moved in with Taekyung so yeah