How can I kill the infected enemies?

[Kedjane]

Are you sure about that? We don't know much about the extent of the operation. Sure, the Archaic dealt with, I'm sure, a lot of alien technology that could potentially be hazardous, but from the sounds of things the Archaic just didn't care about that kind of thing.

What were the researchers really expecting? Did they consider the destructive consequences of their actions? Did they have any inkling that opening up the Tuurngait tomb would have killed pretty much everyone in the mine, and that the destructive influences would have spread beyond in the world?

Yes, Howard was one who was cautious and warned against defilement of the tomb. But judging by how everyone else ignored him completely and started moving everything around proves that--well--the Archaic, for all of their knowledge of history and the alien forces who've influenced the planet didn't seem to be all that well prepared for what they encountered next.

If the Archaic was pompous enough to just open up mines and tombs willy-nilly ignoring the consequences of their actions, then why would they bother to prepare themselves with guns? Everyone was just so sure of themselves, from the observations I've made in the game, that I could easily understand how the Archaic would be that stupid and not bring firearms as precaution.

Well, the Archaic do have some strange logic going around but I still don't think taking a cyanide pill is their solution to everything. I think it sais somewhere (or is at least hinted) that they have encountered hostile threats before.
Since they are a huge worldwide company (20'000 members if I remember it right) they are also bound to have some sort of rival company (or at least some people trying to find them), in which case someone would have to carry a firearm.