Huh. I have as many peeves as the next grammarian, but "slay" doesn't particularly bother me. It's no farther off-kilter than "pix" in the beloved 1935 Variety headline "Sticks Nix Hick Pix" (translation: Small-town and rural viewers aren't responding positively to movies set in rural milieux).
Headlines are often highly condensed -- I've put in my time as a newspaper editor deleting unnecessary syllables to get the danged thing to fit on one line. And "Slaying" is at risk of being read as a participle rather than a noun-adjective -- as in "Slaying Suspect Amanda Knox, Mystery Killer Vanishes."
In any case, I probably don't need to worry that "slay suspect" will turn up in The Atlantic anytime soon.