I said this on my news feed the other day but I'll say it again here. :-) It's not fair to say that the good guys are stupid. :p They aren't aware of the whole situation like we, the audience, are. All the police had to go on is that Tae Sun's fingerprints were all over the first murder weapon, as well as over the house, plus he was found at the scene covered in blood. Logically, all evidence pointed to him. Yes, he claimed he was framed, but really, a lot of criminals will of course say that. They were not stupid for not believing him, especially when his "friends" lied. At that point, he was viewed as a liar so when the guy tried to strangle him in jail, it was easy to assume he staged it and lied about the attempted murder. The second murder, once again, there he was at the scene of the crime. Again, all evidence points to him as the murderer. In fact, even the prosecutor woman thought he was guilty up until recent episodes. They are all actually being very logical in looking at the evidence presented before them. I would have found it more weird if they had believed what he was saying.
Also, I agree. The OS is awesome.