I saw a tutorial on this forum that wasn't very helpful as it was outdated and people still needed help. This is how you install Steam while running Kali 64-bit.

First thing you'll need to do is go grab the latest Steam.

You can find Steam on valve's official website:
http://store.steampowered.com/about/

Installation Instructions

You'll download a steam .deb file. Replace "replace.deb" with your steam .deb filename.

Code:
sudo dpkg -i replace.deb
sudo apt install -f
Now that steam is installed, if you want to run Steam as the root user you'll need to locate /usr/bin/ and find a file called "steam".

Open this, and replace the following lines of code:

Code:
# Don’t allow running as root

if [ "$(id -u)" == "0" ]; then
show_message –error $”Cannot run as root user”
exit 1
fi
With:

Code:
# Don’t allow running as root
if [ "$(id -u)" == "1" ]; then
show_message –error $”Cannot run as root user”
exit 1
fi
Essentially replaced 0 with 1 in this file. This will disable that annoying root message.
If you're running a 64bit version of Kali and you're receiving a libGL.so error upon Steam's startup the following will fix that.

Code:
dpkg --add-architecture i386

sudo apt update

sudo apt install libc6:i386 libqt4-dbus:i386 libqt4-network:i386 libqt4-xml:i386 libqtcore4:i386 libqtgui4:i386 libqtwebkit4:i386 libstdc++6:i386 libx11-6:i386 libxext6:i386 libxss1:i386 libxv1:i386 libssl1.0.0:i386 libpulse0:i386 libasound2-plugins:i386
Now, reboot your system and run steam from the new desktop icon. Done!