Shaun's symbolism comes in the form of the romantic story. The movie, often referred to as a "rom zom com" (romantic zombie comedy), actually serves as a sort of cinematic relationship guide, comically instructing us about dealing with commitment issues. Look at the order in which the members of Shaun's party are killed (killed dead, not undead): #1: his stepfather (Bill Nighy); #2: his mum (Penelope Wilton); #3: the other guy who loves his girl (the underrated Dylan Moran, who must be seen in Run Fatboy Run); #4: his roommate (Peter Serafinowicz); and finally, #5: his immature best friend (Nick Frost). These are the people that have to die in order for Shaun (Simon Pegg) to devote his full attention to Liz (Kate Ashfield). In real, non-lethal terms, they are the people Shaun has to let go of before he can fully connect in a relationship.
Who hasn't had to deal with one or more of these examples when in love? I don't want to get all psychological about it, especially with the mama's boy stuff, but certainly everybody has had that good friend who has to be compromised somewhat because he or she poses a threat to the relationship. The interesting thing is that Ed doesn't actually have to die, whereas the rest of the threats do. The roommate one is a little less obvious than the others, in the sense that he's just representational of the bachelor pad. And there's probably other characters that can also be considered in the order, such as the bartender, who personifies Shaun's penchant for hanging out in a bar and drinking too much.