The fact that the film ends on a happy note rather than the very dark, less enjoyable alternate ending where Lindsay dies. It just fits the tone of the film and the characters better for her to live.
To that end, Goodkat allowing Lindsay to live and going through the trouble to bring Slevin his father's watch before he leaves town. It's by far one of the most amazing parts of the film, though brief, as you realize Slevin is basically the personification of Goodkat's humanity and they're saying goodbye for good. Bit of a Tear Jerker too.