About the bullsquids, maybe it was the Headcrab that killed the bullsquid, although I have no idea, I never saw a Headcrab kill a bullsquid
About the blood on the walls, it may have been a Man-Hack who accidentally killed the metropolices
Most likely the whispers are most likely the Vortys, probably one of the nodes that he had to walk, blocked, and he was trapped in the building