is the word 'the' used correctly in the first sentence. I am not specific in this case. I want to talk about people in general. That's why I think I should leave out the word 'the'. I think if I use the word 'the', it means the people have been mentioned before, which is not the case. But I am not sure. should I Leave the word "the" in the following sentence.
(The) People who want to leave their own societies, for whatever reason, have the right to do so.
It does have a different meaning, and hence are not always interchangeable. Since you want to refer to people in general, you are correct that you should not use the definite article. If you use the definite article, you would convey that you are referring to those people who want to leave their own societies as given by the current context. One would need more context to be able to judge whether using the definite article is inappropriate.
To understand the differences better, consider the following:
(1) The animals who are trapped under the snow are unlikely to survive.
(2) Animals who are trapped under the snow are unlikely to survive.
(1) refers to those animals who are trapped under the snow in the current context. For example, (1) can be used if referring to a particular ongoing situation. (1) can also be used in a more general claim, such as:
(1a) In a blizzard, the animals who are trapped under the snow are unlikely to survive.
Note that in (1a) "a blizzard" is an indefinite reference, making (1a) a generic statement.
In contrast, (2) refers to indefinite or generic animals who are trapped under the snow, rather than a particular group. So a news report about a particular recent blizzard would use (1) and not (2), while a documentary that makes a passing remark about animals trapped under snow would use (2) rather than (1).
It may be worth noting that one special feature of a generic statement is that it can be vacuous:
(2a) Animals who are trapped under a hundred meters of snow are really unlikely to survive.
(2a) clearly does not refer to any actual animals who got trapped under that much snow! And this special case also makes it clear how the definite article would make a difference, because it cannot reasonably be used here.
It's one of those questions about placing a definite article before a noun followed by a defining relative clause.
In practice it makes little to no difference one way or the other. However, when you are referring to a group defined by a relative clause, you don't need the definite article unless that group is somehow unique or is limited by number. Thus, you get a specific (unique) sub-group from a general defined group.
More context would be helpful in making the stylistic choice. You should likely use "People who..." instead of "The people who...", but depending on the context, it could sound natural either way.
