Those that have answered 'no' to this question should be ashamed to wrap such nonsense up in the name of science. As Michael Crichton observed, "Has it ever occurred to you how astonishing Western society really is? Industrialized nations provide their citizens with unprecedented safety, health, and comfort. Average life spans increased fifty percent in the last century. Yet modern people live in abject fear. They are afraid of strangers, of disease, of crime, of the environment. They are afraid of the homes they live in, the food they eat, the technology that surrounds them. They are in a particular panic over things they can't even see — germs, chemicals, additives, pollutants. They are timid, nervous, fretful, and depressed. And even more amazingly, they are convinced that the environment of the entire planet is being destroyed around them. Remarkable! Like the belief in witchcraft, it's an extraordinary delusion — a global fantasy worthy of the Middle Ages."
There are lots of reasons we should seek to explore--and eventually, colonize--other worlds.
First of all, through such exploration, we will learn more about the universe. This is worthwhile for its own sake. However, if you seek external justification, it's simple: the more our science advances, the better our technology gets, and the more our quality of life improves.
Second, politically speaking, colonization of other worlds would help to unify the human species, as any great scientific project does. This is not to say that the result would be a complete unification--of course not--but the more we advance, the more unified we are likely to become. This will help to minimize the negative behaviors mentioned in the question.
Thirdly, we must colonize other worlds because then no single event can end humanity. Even if you assume the pessimistic view that we are a destructive species, you must also recognize the optimist's counter-argument that we are also a magnificent species. We can do amazing and terrible things, and it would be a tragedy to lose the amazing as a result of the terrible. Colonization would prevent any such singular event from killing us off.
I don't think that humans should start 'conquering' other planets, at least not yet. (I used the word conquering because that is how us humans would see it). I don't think that humans should start exploring until we are less greedy and selfish. Once we are more civilized (like they are in star trek) then I think it will be safe for us to explore. Not yet though.
We have over a billion people starving on THIS planet. An AIDS pandemic in Africa and Southeast Asia that kills millions. A serious problem with poverty levels and hunger in America, especially for children. Why on Earth would we need to waste time and resources spreading our nonsense to other planets?
I'm pretty sure we shouldn't.
After all, with our current disgusting state of behavior, I'm not sure it's a good idea we should spread to other planets and continue our nasty tradition.
But then again, hey! We don't have to worry about this! After all, we have nuclear weapons pointed at ourselves as well as no discipline! I'm pretty sure we won't even be around in 500 years because somewhere along the line some politician's going to press the big red button and our whole planet of life is going to kill itself with nuclear weapons it didn't have the discipline to not use! :D