I think “Ismail” is the arabic spelling of “Ishmael.”
To the contrary of other posters, I don’t think this means he is NOT islamic.
I also think the name of “Bush” being involved might be a coincidence. The character’s name is “Ishmael Bush,” and HE is the one with the axe. I think this might indeed be the intended reference, though, because the axe figures prominently, apparently, in the story, and is referred to as “Ishmael’s axe.” That the gunman used the arabic spelling must mean something, too (to the extent anything can mean something to a deranged person).
Yup...
You are right. The two meanings and spellings are not mutually exclusive. English majors and creative writing majors love puns and double meanings. It all fits. Actually, it's clever. Evil, but still clever.