You might haven't noticed while playing, but this kind of graffity can be seen in the game from time to time, on the city walls. You can see it for a brief moment in the fan-made trailer, tho (the link is above). The drawing conveys the message that can be applied both to irkens and the Combine, I believe.
And they do have a lot in common. Both irkens and the Combine are heavily military-oriented, live abiding to the certain rules, have no individual freedom, aren't allowed to reproduce naturally, etc.
That's why I decided to draw this sad, a kind of crossover, fanart.
Half Life 2 © Valve
Irkens © Jhonen Vasquez, Nickelodeon
Brushes used by Kecky
Artwork by Firey-Flamy