All octopuses have poison, but few are fatally dangerous. The large blue-ringed octopus, however, is considered one of the most venomous animals known; the venom of one is enough to kill ten adult humans. It uses the neurotoxin tetrodotoxin, which quickly causes respiratory arrest. The bite of the Pacific Giant Octopus will not only hurt, but it will also inject poison into its target (although this poison is not deadly).
Although octopus and squid are formidable fighters in nature, they are usually not dangerous to people. That doesn't mean they're always harmless. Some species are especially well equipped to defend against larger creatures, and are strong enough to kill a human if they feel threatened. Some octopuses have venom that can even be deadly to humans, but attacks are rare.
In fact, wrestling with octopus was actually a sport in past decades, although it has since been banned in places where it was popular, such as Washington State. Of course, there are a number of species ranging from scary big guys worth fighting for to this irresistible little guy. A blue-ringed octopus is one of the most dangerous animals in the ocean. Its bite can be fatal to humans.
However, very few people have died from the bite of a blue-ringed octopus. These octopuses are not aggressive and tend to stand alone unless attacked. To make a long story short, no, most octopuses can't kill you, but that doesn't mean you shouldn't be respectful and extremely careful during your encounters with them. However, if a real octopus could kill a human using pure force, it was most likely a giant Pacific octopus.
If this doesn't disgust you, there's another reason not to try to swallow a living sea creature that could kill you. The saliva of the giant Pacific octopus contains the proteins tyramine and cephalotoxin, which paralyze or kill prey. Humboldt squid has a particularly notable reputation for being dangerous to humans, and is sometimes billed as a ruthless man-killer who swarms and devours humans in large groups.