Since one kilometre has no volume, it cannot technically contain any venom; none.
You must mean kiloGRAM, not kilometer? If 1 gram kills 150 people, then 1 kilogram kills 150,000.
Platypuses do not kill people. Their venom can cause agonising, almost paralysing pain, but it cannot kill a healthy human being.
30 FYI venom
Yes, many animals can kill humans through sheer strength or through toxic venom.
Many types of spiders have venom that can cause serious health problems in humans. The venom of some species can kill if not treated.
The average dose delivered in a bit is 44mg. Although the venom is highly toxic - there have been no recorded fatalities of victims (usually herpetologists) dying. There is a highly effective antivenin available.
It is estimated that vipers, which include several venomous snake species, kill several thousand people globally each year. These deaths are largely attributed to the severity of their venom and the areas where these snakes are commonly found.
The LD50 figure (the quantity of venom needed to kill 50 adult mice) is 0.25 µg/kg (50 micro-grammes per kilo)
All cobra are venomous. Death would depend on the quantity of venom injected and the health of the recipient. But certainly many people are killed by snakes in the tropics each year. If medical aid and anti venom is provided within minutes of snake bite, chances of saving the victim are pretty high. If it is a king cobra that bit, the venom is very potent and can kill the victim within 5 mins.
A Black mamba bite can easily kill 10 full grown men. 15-20 mg of mamba venom is enough to kill an adult easily. A black mamba injects around 150 mg of venom in every bite.
There are 133 people per square kilometer. (2008 estimate)
1,425 people per square kilometer in Cape Town