I think that his harmless appearence is to deceive the audience into thinking he is the only normal guy, that is right until the end parts of the fim.
SPOILER
I know that in the script we read we know from the beginning of the film that HE IS a serial killer, an accountant in real life. But in a recent plot summary, it was revealed that Edwin is a phycician (Sorry for the spelling), and is described as the only non killer, which leads me to believ that we wont know of his Serial killer nature.