kwin_killer_helper: 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/kwin_killer_helper - from package: kdebase-workspace

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

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