John Wick was a great movie.  John Wick 2, less so.  It's still enjoyable, but I think one very simple line change could have made a significant improvement.

In the bar, when John was drinking with Cassian, and John explains why he did what he did, we get this exchange (paraphrased because I don't remember the exact quotes):
Cassian: I see.  You had no choice.
John: Right
Cassian: Well, I still have to kill you.  You know how it is, an eye for eye.  I'll make it a quick death.
John: Thank you.  I'll try to do the same.

An eye for an eye??  In a league of assassins?  Nope sorry I don't buy it.  If Cassian was to go after the person who ordered the death maybe, but to revenge against the tool that did the job?  He's ONE of them!  He KNOWS he has no choice in the matter.  This entire subplot can be fixed with one simple line.
Cassian: I see.  You had no choice.
John: Right
Cassian (pained): Well, I don't care.  I'll make it a quick death.
John: Thank you.  I'll try to do the same.

This then serves dual purpose.  Cassian's actions are then tied to his emotions, not some assassin code (that seems counter-productive).  Further, his disregard for the rules of the job (as John had no choice) can act as a foreshadowing of John's later actions.

One line, better movie.