After Goodkat shoots Lindsey, it seems like Fridge Logic that he doesn't bother to check whether his victim is truly dead. But here's where the Fridge Brilliance kicks in: most likely Goodkat does see that Lindsey is still alive, but seeing this makes him realize that Slevin had warned her about the hit. At that moment Goodkat knows that Slevin must really love Lindsey for him to disobey his mentor like this, and that's what makes him spare Lindsey: he can relate to Slevin's feelings for Lindsey, because similar empathy made him spare Slevin 20 years ago. So after processing all this in his head, Goodkat slips away from the morgue, letting Lindsey think she has fooled him. This also explains why, after Slevin asks Goodkat how he found about Slevin's plan to spare Lindsey, Goodkat just says: "I'm a world class assassin". A world class assassin wouldn't just shoot someone once and leave, without checking that the victim really is dead.
Though it takes a second watching to pick up, there are 3 very subtle clues as to Slevin's identity; 1. Henry is seen playing with a baseball mitt and is promised a trip to a baseball game; 2. A bookie is killed by a very fast, very well aimed baseball, suggesting the killer is probably a big fan of the sport; 3. When Slevin is told to play ball, in a quick and literal-minded fashion, he quips "You think I'm tall enough?", suggesting he has a ball sport on his mind.
Whenever someone tells a true story, it is shown in flashback at the normal frame rate. The Fridge Brilliance is as followed: When it comes to Slevin's recounting of how "bad things happen in threes", the frame rate is slightly different, having a shuttering effect as he recounts being fired, being evicted and his girlfriend having an affair on him. However, the frame rate stops having the shuttering effect and goes to the normal frame rate when he is punched (which turns out to be the only true part of the story we find out later). And during your first watch through, no one would notice it. It is only after you rewatch the film does it truly stand out in comparison to the other flashbacks.
At first glance Goodkat seems lucky to be able to actually sell to The Boss the preposterous idea of using some gambler in debt as a hitman. And then we learn that Brikowski was both a heavy gambler and the one who killed Slevin's mother for The Boss and The Rabbi...
The men who pull the strings are driven by revenge for the death of a family who owed money to gangsters, which may seem a "righteous" thing to pursue given the police corruption that Slevin points out. However, they themselves kill the innocent Nick Fisher who owed the same gangsters money, family members who were probably innocent (the sons), lots of goons (some of whom are possibly relatively harmless as seen when they pick up Slevin, or who possibly just owe the gangsters like Max did), and the police detective who possibly redeemed himself like Goodkat did.
However, the above may be averted when you take into account, Nick Fisher is said to have gone to jail for sexual assaulting a 14 year old, and the gangsters who are targeted for murdered were okay with the murder of a completely innocent family just to send a message. They even hired an assassin to kill the kid because no one else wanted to do so. So, in a way, it's an Evil Versus Evil scenario that reduces the above mentioned fridge horror.
What is the advantage of having a deceptively tall knock? You can avoid some nasty surprises.
