Is it really a thin line? A line that anyone might cross if pushed hard enough? Even someone considered to fight for and protect the good and innocent? It really is tough to say - although I sort of think and would like to believe that Donnie Yens character would not fall into that trap of going over the top or crossing the line.
Benny Chans last movie - a really good one. And apart from Donnie Yen (also action choreographer and if you know him, I need not say more), it is especially the very charismatic Nicolas Tse that lifts this movie up. You almost root for him too. At least you can feel his pain.
There are vibes of Heat here, especially in one big action set/shootout. And that is meant as a compliment to both films. Still Heat had more gravitas if you think about comparing them. On the other hand, this has more fights and action set pieces ... and really good ones at that. And that is what this movie can boast with ... if you're a fan, you might not even care about story issues or certain characters not getting more screen time.