Titled it that way to as not to spoil it for anyone who is still going to see it.
Mr. Orange is the worst undercover cop ever.
1. He killed a civilian. Yes, she had shot him first when he and Mr. White went to car jack her, but shooting her in the chest and killing her instantly was bad, even though you can tell he instantly crushed him (the look on his face said it all). Sure, it's likely that Mr. White kills her from behind seconds later if he doesn't, but still, a trained cop should know better.
2. He did a poor job of fitting in. In the opening scene of the film, he barely said anything, while everyone else chatted away.
3. He had a terrible poker face. After Mr. White killed those cops during their getaway, as they approached the train tracks right before the car jacking, his body language screamed COP. He was clearly deflated and stunned by having just seen Mr. White gun down several cops, which is a normal reaction of anyone with a soul of course, but if you are going to be an undercover cop in a caper like that, you have to act a little better than that.
Okay, calling him the worst undercover cop ever is a slight exaggeration, but it makes for a good start to talk about this great film.