Just rewatching the 1960s Vincent Price version of I Am Legend (The Last Man on Earth), and in the first minute or so we see him look at dead bodies and he narrates, "They feed off of weak ones."
I do not recall this from the book (which I have admittedly not read in years), and actually the line in the film surprises me, which shows how I have forgotten the film also. In Smith's I Am Legend he never says this, but it seems plausible that the Darkseekers would indeed eat each other, although we don't see it happening.
My question is simply whether the book has the idea of vampires preying on each other or not. And if it has the same line that Price used, is it possible that he was wrong? This is very unusual behavior for many vampire/zombie stories; the norm is that somehow the infected recognize each other and do not deliberately attack other infected.