I was stealing corpses for the sake of retrieving samples of the viruses that they had in them, usually. I did it to Birkin and someone that I don't recognize for that reason. Krauser, I suspect, was similar; I wanted to verify his death for myself and once I had done that, I took the parasite from his body.

...I'm not entirely sure what I was doing with them, granted - whether I wanted them for myself or to sell them to other people. Seeing as I was running a fairly large operation at one point, I must have had contacts that I could sell bioweapons and viruses to; I also wasn't unfamiliar with using them on myself, if my actions with Uroboros and whateve I'm infected with now are any indication. Las Plagas in particular could be used to do terrible things; I know for a fact that I did use them to do a few things that were rather horrific.

I assume Krauser infected himself for the same reason any of us did. The power.