Complex sentence: He approaches and tells the busboy, "it's OK", and then begins to leave again; the busboy visibly relaxes, and Phillip shoots him in the head (presumably for betraying himself as a witness).
Split into simpler sentences: He approaches and tells the busboy, "it's OK", and then begins to leave again. The busboy visibly relaxes, and Phillip shoots him in the head (presumably for betraying himself as a witness).