It seems killall has been removed from the default toolset on Red Hat distros (i.e. CentOS and RHEL). The logic is sound: as you probably know killall does potentially dangerous different things on other UNIX variants; chiefly it will kill every single process on Solaris/SunOS with wonton disregard.

The preferred weapon of choice is now pkill, as in:
# pkill victim
...where victim is the name of the process(es) you wish to terminate.

But if you're like me, you don't use Solaris and for nigh on 20 years you've been conditioned to reach for killall when you need to quickly hunt down and slaughter a runaway process on your linux hosts.

Creatures of habit die hard and in old age ought be afforded some luxuries, thus:
# yum install psmisc
and
# killall thepetulantbastards

For more of my favourite packages to install on a fresh RedHat, please check out my Favourite RedHat Software.