Dude, stop being a troll or gtfo.
But on the off-hand chance you really do think that?
Undercover cops are a real thing, and they have to witness crimes all the time, sometimes even to the point of being an accessory. Granted, outright murder isn't usually on the table, but shit happens sometimes, especially when setting up sting operations for high-value drug deals, just like the one they got cooking in this manga. The undercover cop can't say a damn thing to anyone that has even the most remote chance of blowing their cover.
But hey, you don't gotta take my word for it.
First, have an article from Business Insider about how real life undercover officers operate.
https://www.businessinsider.com/what-life-is-like-as-an-undercover-cop-2014-4
And here's a PubMed article on the psychological stresses of being an actual undercover officer.
https://pmc.ncbi.nlm.nih.gov/articles/PMC12346469/