It is not that I hate headcrabs, it is that I am not sure what the logic is to them.
Why do they exist? You never see what they zombify in Xen in HL1. Even Opposing Force, which was partly to expand the world of Xen, all you see is the human-zombies grow bigger and nastier. HL2: more variety is introduced (and some variety removed) but the targets are all humans still. For some reason, an alien species evolved on another world specifically to zombify humans on Earth when Gordon Freeman ripped a hole in the space-time continuum? I have seen no suggestion anywhere that they zombify any of the Xen critters so why do they exist? So annoying! The only viable explanation I can think of atm is that something planned to attack the Earth and for a number of years before Black Mesa, they let the headcrabs run loose in Xen where they are mostly harmless. Perhaps the G-Man is actually an alien and he happens to like looking at the the inside of human chest cavities?