Of all the many faults Hythloday finds with European society (there are lots!), pride is numero uno. At the end of his description of Utopia he spends a lot of time explaining just how bad pride can be:
"And in fact I have no doubt that every man's perception of where his true interest lies... would long ago have brought the whole world to adopt Utopian lows, were in not for one single monster, the prime plague and begetter of all others—I mean Pride. Pride measures her advantages not by what she has but by what other people lack. Pride would not deign even to be made a goddess if there were no wretches for her to sneer at and domineer over. Her good fortune is dazzling only by contrast with the miseries of others, her riches are valuable only as they torment and tantalize the poverty of others. Pride is a serpent from hell that twines itself around the hearts of men, acting like a suckfish to hold them back from choosing a better way of life. Pride is too deeply fixed in human nature to be easily plucked out." (2.109-110)
Yowza. Hythloday—not a fan of pride, the sin of thinking you're better than everyone else.
Hythloday thinks this sin is particularly horrible because it needs other people to stay unhappy or miserable in order for you to keep feeling good about yourself. Considering how much time Hythloday spends criticizing the poverty and oppression going in Europe, it's not surprising that he would be so heated about what he think is its root cause.
We've also noticed that Hythloday is so worked up about this pride issue that he suddenly starts using very poetic language instead of his usual straight-forward, factual descriptions. The most obvious poetic aspect of this rant is his use of personification: he's turned the abstract concept of pride into a creature with characteristics of a living thing. Ta da! We bring you Pride, the "monster," "goddess," and "serpent" (hint: you can often tell something is being personified when it's capitalized).
This image of pride is clearly so important to Hythloday because he really wants his listeners to remember it—that's why he uses such strong, imaginative language. It sticks with you, right? It's a lot easier to understand the danger of something if you describe it as trying to strangle your heart (yikes!) than if you explain the moral, social, and economic consequences of a particular tendency in human character (snooze).
While Hythloday's point of view here may not sound like rocket science to us (pride makes us want to accumulate wealth and keep it ourselves... obvious) it would have been a more radical perspective at the time. For much of the Renaissance and before, a person's economic status was linked to their spiritual and religious status: God likes good people and gives them wealth, less good people don't get wealth. Seriously. Economic injustice was considered an oxymoron. So give Hythloday some credit. Even though his rant about pride may sound a bit old-fashioned, he's actually being kind of forward-thinking.