Lets see, on my HDD I have
bikini.exe
California.exe
StressRelief.exe
9Coronas.exe
frog_blender.exe
fly.exe
gerbil.exe
joefish.exe
lemmings.exe
I guarantee you, that most people would think frog_blender.exe is in poor taste and quite sick, some people think its funny. I think the most likely explanation for the killpresidentbush.exe file is that is of similiar nature. And of course, for obvious reasons, it would only be distributed among a very select crowd. I'd refrain for jumping to conclusions, in that possession and distribution of such a file in and of itself would not make one a terrorist per se. Furthermore, any of the executables that I have, could be named anything I want.
Of course since I don't know what the file in question does, this is all pure speculation; not much different from what you all are doing I might add.