It used to be pulled in as a dependency of GConf2, but it was recently
removed from there[0]. Now on regular Fedora Workstation install it's
being pulled in with GNOME Boxes:

liveuser@localhost-live ~]$ rpm -qf /usr/bin/killall
psmisc-23.1-5.fc30.x86_64
[liveuser@localhost-live ~]$ rpm -e --test psmisc
error: Failed dependencies:
psmisc is needed by (installed) glusterfs-fuse-6.0-0.1.rc0.fc30.x86_64
[liveuser@localhost-live ~]$ rpm -e --test glusterfs-fuse
error: Failed dependencies:
glusterfs-client >= 2.0.1 is needed by (installed) libvirt-daemon-driver-storage-gluster-5.0.0-3.fc30.x86_64
[liveuser@localhost-live ~]$ rpm -e --test libvirt-daemon-driver-storage-gluster
error: Failed dependencies:
libvirt-daemon-driver-storage-gluster = 5.0.0-3.fc30 is needed by (installed) libvirt-daemon-driver-storage-5.0.0-3.fc30.x86_64
[liveuser@localhost-live ~]$ rpm -e --test libvirt-daemon-driver-storage
error: Failed dependencies:
libvirt-daemon-driver-storage = 5.0.0-3.fc30 is needed by (installed) libvirt-daemon-kvm-5.0.0-3.fc30.x86_64
[liveuser@localhost-live ~]$ rpm -e --test libvirt-daemon-kvm
error: Failed dependencies:
libvirt-daemon-kvm is needed by (installed) gnome-boxes-3.31.90-2.fc30.x86_64
[liveuser@localhost-live ~]$ rpm -e --test gnome-boxes

but as the Boxes are expected to be distributed as Flatpak, they are not
in the default Silverblue installation. Thus it will be better to be
explicit about the killall dependency.

[0] - https://src.fedoraproject.org/rpms/GConf2/c/6668eba?branch=master