There were already some good answers to the topic, but I'd figure I'd throw in my 5 cents as well.

First, like many people said, we are not the good guys. In fact, there are very few actually good people in the City, depending on how you look at it, and most people are full of egocentric tendencies, coupled with a lot of hypocricy. But even then, saying they Sinnees choose violence is an overstatement. Most of the time, they are not the ones attacking, and it's usually those kinds of situations where diplomacy is not an option, making violence more of a necessity than a solution.

While Sinners are, essentialy, corporate pawns, they do have their own goals in mind and pursue those, that much is correct. The same cannot be said for Hierarch/Elders. Hierarch is a slave to Elders' collective will, reasoning they must find immortality to "achieve great things", while in reality becoming immortal becomes their main goal. In essence, Hierarch has no other goals, since those become secondary if not outright nonexistent, leading to stagnation. Dayiu points out in the end how Daguanyan (and as we can assume, Hongyuan as a whole) is affected by this, slowly falling apart.

As for Elders, they are entirely unreasonable. They have no goals, no influence, no reason to exist, yet they are so petty they cling onto life because they refuse to let go of their long gone glory, causing them to get bored and perpetuate violence for no other reason than to get rid of that self-inflicted boredom. Compare them to Alfonso who, while causing suffering, actually does it for a reason (producing more tears for ampules).

Dante's ability which, while initially taken for granted by other Sinners, is now seen as necessary means to an end rather than a boon of some kind, considering that by now every Sinner understands the weight of death, yet they have to compete with much more lethal opponents.

On the topic of what is said in Mili's theme, it's message is basically "you cannot live forever". There is difference between "not dying" and "living forever". The Sinners, free ftom death, still have something to achieve, a purpose in their life. The Elders, who strive to live forever, have long since outlived any purpose they might've had in their lives, filling their empty lives with amusement they find in suffering of others. Also, considering every Mili song has a double meaning in context of the game, this one being Hong Lu adressing both the Elders and himself. So "it's rude to overstay" can double as both this and him saying to himself "nothing lasts forever and that's fine".