I launched steam as normal, it updated. It also updated tf2 after which I proceded to playong the game. The game started as normal and I joined a serve. After the loading screen was over the game froze. I went on with killing the game and then I noticed that in my root directory (steam is installed in /home which is in a different partition) I lost ~8GB worth of space.
This is what dmesg shows:
[ 659.457863] hl2_linux: segfault at 0 ip 00000000f76251a0 sp 00000000ffe4ac48 error 4 in libc-2.19.so[f74f5000+1bb000]
I deleted some files from /var/lib/systemd/coredump but those where only about 3.5GB.
What could be hogging my HDD space?
More information is needed to even begin to answer this question.
So tf2 crashed and somehow 8GB more of the root partition are being used.
What makes you think the problems are related; is the root partition completely filled? How did you notice an extra 8GB were used?
If the increase in disk usage and tf2 crash are related, we need to know what files suddenly used up the space.
Use this command, modifying the mtime and size as appropriate, to find the new files taking up disk space on the root partition:
find / -xdev -type f -mtime 5 -daystart -size +10M -printf "%p %k KB\n" | column -t
Also the /var/lib/systemd/coredump files probably were made when tf2 froze or some other process that may have caused tf2 to freee.
So instead of deleting coredumps you should use them to debug the program.