Why are all spiders venomous?

Spiders don't only eat blood.

A lot of spiders inject liquefying venom into their prey, and its insides turn into a sort of fluid which can then be easily sucked out by the spider. They then discard whatever is left. This is mostly spiders that live in webs, and some jumping spiders.

Bigger spiders, such as tarantulas or other roaming spiders use their fangs to crush up their prey while spewing digestive, liquefying juices over it, and eat up the liquefied insect. They usually only leave a mushed up, black ball that used to be the insect.