While replaying HL, I got to We've Got Hostiles and I met a weird scientist in one of the warehouses... When I talked to him, he died immediately without saying a single word! A heart attack?
And when I talked to his corpse, it said: "I'm slowing you down, am I?" - of course you are! After that he was silent tho.
So I reloaded my game and ignored the scientist, going up a stairs instead. When I was on an upper level, he died suddenly for no reason... Maybe he was scared that I was leaving him alone and died of fear.