Learn English – What does “peg” mean in this sentence

"It's not intuitive" in the context of user-experience design refers to intuition:

...
4. the quality or ability of having such direct perception or quick insight.
...

Essentially, an experience being intuitive means that a user can walk up to the interface and immediately understand it.

The passage you quoted is describing the tendency of users to not admit difficulty understanding an interface, but it's pointing out that if a user has difficulty understanding it, it's really at the fault of the designer. Design should be easy to understand without too much digging--it should be "intuitive."

Another good term to describe an interface like that is "discoverable." In other words, users can discover features without having to read manuals or help documentation.

In this context, I think the author is simply saying:

they’ll go live recklessly and behave dangerously

The reason we might tell someone to "go play in traffic" when they are annoying us is that we want them to go away and not come back. I think the phrase is used in jest; if taken literally, it's almost like we are saying that we hope they will go get seriously injured. (I'd recommend most learners avoid the go play in traffic idiom.)

Similarly, and eight-ball of cocaine is a pretty large amount (3.5 grams), and according to one website:

The estimated minimal lethal dose of cocaine is 1.2 g, but individuals with hypersensitivity to cocaine have died from as little as 30 mg. Still, this is usually not the case with cocaine addicts, who develop a high tolerance to cocaine in the central nervous system. In fact, some cocaine addicts with considerable tolerance have reported that they can tolerate up to 5 g of cocaine daily.

So, your sentence begins with:

This truth [that everyone will die] will liberate them from all responsibility, and they'll go snort an eight ball of cocaine and play in traffic.

In other words, because they feel like nothing they do will prevent their ultimate, eventual death, they simply won't exercise caution while living. The author could have just as well said:

This truth will liberate them from all responsibility, and they'll go take lethal doses of drugs and do very dangerous recreational activities.

However, that paraphrasing is more bland than the original. I don't think the author is using established idioms per se, but rather simply furnishing some colorful examples.