kill_instance: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/kill_instance - from package: python2-boto
/usr/bin/kill_instance - from package: python-boto

You may install the required package with command yum install {package}

Error in other languages:
kill_instance: bevel nie gevind nie
kill_instance: comandă negăsită
kill_instance: comando não encontrado
kill_instance: commande introuvable
kill_instance: command not found
kill_instance: käsku ei ole
kill_instance: không tìm thấy lệnh
kill_instance: komanda nerasta
kill_instance: Kommando nicht gefunden.
kill_instance: kommandot finns inte
kill_instance: komut yok
kill_instance: nie znaleziono polecenia
kill_instance: níor aimsíodh an t-ordú
kill_instance: no se encontró la orden
kill_instance: no s'ha trobat l'ordre
kill_instance: opdracht niet gevonden
kill_instance: parancs nem található
kill_instance: perintah tidak ditemukan
kill_instance: príkaz nenájdený
kill_instance: příkaz nenalezen
kill_instance: команда не найдена
kill_instance: командата не е открита
kill_instance: 命令找不到
kill_instance: コマンドが見つかりません