Remember that Slughorn is a slitherin. Whether or not Snape is evil or not, but he killed Dumbledore, who trusted him. If Dumbledore didn�t know about Snape to turn on him ( I don�t know about that, I personally think Dumbledore "told" Snape to kill him to keep his cover ) but if he didn�t know and trusted Snape, then perhaps he mistrusted Slughorn as well. Who knows what he could have done while everyone else was fighting. He could have helped the Death Eaters after all......