In their view it's better to be a big fish in a small pond than a small fish in a larger pond. Of course this isn't a foolproof strategy (dictators and kings sometimes come to ignominious ends) but holding onto as much wealth and power as possible while keeping the majority of people disenfranchised is a much safer strategy than spreading the benefits as widely as possible and hoping that everyone will love you enough to assure your future security.
Well a lot of people disagree about what things like quality of life and adequate environmental care are. One of the beauties of capitalism is that it allows people with wide disagreements about what's important in life to coexist and even benefit each other.
Yes, but it doesn't assure that, and that's where it gets ugly. If too much of capital is controlled by someone who doesn't care about the environment and is fine with cutting corners or actively polluting, then the less well-off people suffer and die, sometimes horribly. If that problem gets bad enough, then everyone could suffer as the whole environment is wrecked - like the population of a country whose leader foolishly enters a war and gets bombed, or a world where reckless fossil fuel production/use continues past the point of long-term sustainability.
You'll recall that Keynes said 'in the long run, we are all dead.' Capitalism would be great if everyone lived long enough to have their day at the top of the pile, just as it is enjoyable to play Monopoly as a board game because while you might lose one game today you might have the pleasure of winning tomorrow. But people are not economic abstractions; if you're poor and you can't compete effectively in a capitalistic economy then there's a good chance that your life will be terrible, and then you'll die.