A less-than-human-becoming-human journey.
One of main characters in both movie and drama is a very similar dark (scary, but also kinda... cute?) psycho with some deep issues and history of psychological (mis)treatment. Both stories show us a lot of violence and you can't really figure out the psycho character till the very end.
Difference between these two is mostly that Bad Guys is a drama and not a movie (so not as short and has more story parts intertwined) and has other significant and complicated characters aside from the psycho.