Quick question about evolution

[Yoo]

I think the confusion here comes from the idea that evolution has a goal. This is false.

Evolution happens when a change makes a species more likely to survive and successfully reproduce. There's no conscious decision to make venom stronger in a species. If its specific venom just happens to help it survive longer than its relatives, then its specific strain will become more dominant.

I pride myself in thinking I understand evolution quite well, and I know that it's not part of the theory that evolution has a goal in mind, and there's no reason to suppose it has. 
The basic question is already answered by Alex K, there's just a few little things left unanswered for me now.

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I've read about arms races, didn't think of that though, you're porbably right. 
Some thoughts: 

Because the prey is usually very small, the amount of venom they get is a lot relative to their size, so even though they are better adapted to handle it, they react just as strongly to the venom as we do...?

What does a poisonous predator have to gain from very weakly poisoning a prey, like the earliest spiders probably did? Is that really a strong enough pressure for the first incidental venomous spider to have a real advantage?

Btw, will make an intro!

Hi there. Evenomation is not a constant; sometimes there will be a lot, sometimes, not very much, and other times, not enough. It's a trade-off. It can range from about a 1 mg to 1 gram for a rattlesnake, for instance, depending on several factors--age, size, body condition, time since last bite, etc.
Venom is a super interesting thing to study. There are some venoms from spiders and snakes that contain proteins with a whole spectrum of activity. If you fractionate it, you can get venom specific to insects, or mammals, or birds, etc. Venoms can contain proteins that specifically target blood clotting, or some nerve impulse transmission mechanism, or several other things. Some animals have more of a certain mechanism to interfere with than others. What kills a person may only cause a dog momentary paralysis. Black widow and brown recluse spiders, for instance, typically aren't as dangerous in a dog as they are in a person.

Thanks for your answer! It's actually quite funny how the venom intended for killing a dog ends up killing us, while "only" paralyzing the dog. The snakes have lived with dog(/like) animals so the dogs are better adapted to it. But superficially it seems like evolution made the venom weaker!