don't really understand the point, I guess maybe they really thought they might get cancelled.
i mean if the Chief could take out all but one of the hitmen by himself then if Castle and Beckett stay they could of killed them all and then the Chief wouldn't have to die and *Beckett wouldn't have to fake her own death at the funeral to get undercover to catch the head bad guy, because he thinks she's dead. It looks Beckett didn't even tell Castle and got him to say 'i love you' because he thought she was really dead. dirty trick*
*all speculation on my part, of course