StingingVelvet: I thought it was Simons you had to kill. If you avoid him at the ocean lab he will reappear at area 51.
An interesting little piece that I've not heard anywhere else:
You can get away with not killing Simons by employing a well-timed 20mm HE shot. If you can trigger the impact at the exact moment the dialog kicks in, his body will fall to the ground, but his 'ghost' will remain. I can't remember if it attempts to chase you or if it remains still, but you cannot harm it. If you ghost Simons at the Ocean Lab, he will come back at Area 51 like nothing happened--and you can do this again there! Very interesting..