tbf, thats an existing bash-complete-ism, fzf.bash just makes it cooler.
To me, bash-completion is horrible. I am looking for a tarball to unwind, and I only remember that it it started with "foo-" and it sure did not end in ".tar" (trying to save keystrokes, y'know). Trying C-i does not pull up anything. Argghh. I am on a machine with bash-completion!
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I often hit C-a then "a" to get an invalid command (still saved to my hist), or C-a C-k if I think there is nothing else going into the kill-ring. bash-completion absolutely messes with me.
If it works for You, that is great. I really dislike it.
fzf.bash just turns `kill` into `ps (args) | fzf`, and does not yet have an equivalent for `killall`.