Interesting characters don't always get the stories they deserve. To wit, Melford Keane, the ''Ethical Assassin'' of the title, a post-Marxist vegan hitman with a passion for animal rights, a penchant for philosophizing about ''the way in which culture produces the illusion of normative reality,'' and a morally dubious only-kill-people-who-deserve-it policy. Alas, this bleeding-heart vigilante is but a supporting player in a twisty yet familiar caper about a young door-to-door book salesman who gets messed up with a vicious crank-dealing conspiracy in a trailer-trash Florida backwater. You get the sense that a first-rate writer (David Liss) has made the mistake of falling in love with a mishmash of second-rate ideas. In the process, a clever character gets stranded in a swamp of silly Southern stereotypes and semi-lurid pulp.