You have to define your terms first. What do you mean by imperialism? I would define it as an attempt to further a nation's interests by establishing military, economic, or political control over other nations.
In which case, there are two main arguments against it: historical and moral/philosophical.
Historical argument: The main example of imperialism in the past 500 years has been the colonialism practiced by the European great powers over large territories in Africa, Asia, and the Americas. These colonial/imperial relationships were characterized by oppression of the people and rapacious exploitation of natural resources. In almost every case, nationalist resistance movements formed and were brutally crushed for generations before they succeeded. Looking back on the historical effect of colonialism on the colonized areas, the result has been almost universally disastrous.
Moral/Philosophical argument: Every person has the right to self-determination, and by extension this leads to a nation's right to form a social contract and govern itself. Thus, it is wrong for one nation to unilaterally invade another for its own benefit.
Or, to take a page from Kant, you must only treat people as ends in themselves, and not as means to your own ends. Imperialism, by definition, subjects an entire nation and uses it as means toward the ends of the imperializing country. Wrong.
I know these are somewhat simplistic, but this seems like an open-and-shut case to me.