"Violence is what people do when they run out of good ideas."
This quote, which forms the thematic core of the novel, comments on the nature of humanity (and Martians). When people get scared, they turn to violence as a way to cope with their emotions. Violence, in other words, is instinctual. When a person runs out of logic, they turn to something that is largely illogical: violence. And that is the point. Violence is, according to the novel, never a good idea. Violence invariably leads to significant consequences for everyone involved in it.
"Heroism is a label most people get for doing shit they’d never do if they were really thinking about it."
This quote describes what Holden sees as the mindset that heroes have. In Holden's eyes, heroes act without thinking about what they will be doing (and the consequences of what they will be doing). They don't think that they could lose their lives; instead, they see something which needs to change almost instantly and equally quickly act to resolve it. They are doers. They just don't think. But that is good, Holden and the novel argues. Otherwise, people wouldn't be heroes and the world would be worse off for that.
"Nothing ever killed more people than being afraid to look like a sissy."
In an effort to not be looked down upon as a "sissy" by their peers, people are more likely to try and impress people. To that end, if they see someone that needs to be killed, they will more likely than not kill that person against all logical thoughts so that they look good to those around them. This quote shows how people are more interested in impressing others than they oftentimes are in self-preservation.
