Zyklon B can be used to kill humans; nearly any pesticide can, when you think about it.
.. on a side note.
See, I hate reading books about the holocaust. I read 4 through school and I got the idea the first time. (Incidently, I would like to comment: no school ever gets events on those right. I should have just ignored it but whatever..)
But on the other hand, the military and political aspects of WWII fascinate me. And any of those books ever gives you a good general understanding (And in sometimes a too good specific understanding) about the Nazi's policies re: undesirables.
So generally speaking, anyone who's done research beyond what you learn in the K-12 system (which is watered down due to the sheer mass of details you need to know in general) will understand.
Apparently even believers (or I would like to call de facto believers) don't know their facts.
It's not "can". That is the "fact" of the Holocaust. I'm going to defer to the experts:
http://www.ushmm.org/wlc/en/article.php?ModuleId=10005189
During the summer and autumn of 1941, Zyklon B gas was introduced into the German concentration camp system as a means for murder. At Auschwitz I, in September, the SS first tested Zyklon B as an instrument of mass murder. The "success" of these experiments led to the adoption of Zyklon B for all the gas chambers at the Auschwitz complex.
http://en.wikipedia.org/wiki/Zyklon_B
Zyklon B was the trade name of a cyanide-based pesticide infamous for its use by Nazi Germany against human beings in gas chambers of extermination camps during the Holocaust. The "B" designation was one of several, and indicates the concentration.
I actually did know the name of the gas was Zyklon B before, but I still didn't know it was a pesticide and it was used for that purpose. This was also before the modern internet was invented so forget even Wiki. Netscape wasn't even around. Yet people still wouldn't know if they don't look for it.