Why is arsenic toxic to humans?
Arsenic is toxic to humans due to the fact that it imitates phosphorous almost identically in its bonding patterns, as both elements are in the same group.

However, why would this have any affect? The most likely reason for this is due to the fact that when the phosphorous is substituted by arsenic in many of a cells vital molecules such as DNA and other phosphorous containing groups, the properties of the new substance is slightly different in terms of its polarity, reactivity, etc. Because the proteins and enzymes within a cell interacts with these phosphate containing groups in such a specific manner, these slight irregularity in chemical nature can have detrimental effects (you can think of the processes of these proteins and enzymes between the phosphate as clockwork, i.e. if the parts are slightly different, it will not work).

I am not as sure about the chemistry of antimony, as I have not researched that topic greatly, but I would guess it being in the same group adds to its toxicity.