From your point of view, to what extent were historical European imperialists justified in their attempts to change people from other countries?

It is important to remember that European imperialism was not merely about overtaking other countries, but also about exploiting a land's natural resources for profit without returning any of that revenue to the people native to the land, or to the former slaves left behind. This exploitation often resulted in economic devastation. For example, some islands in the Caribbean have been left more vulnerable to natural disaster due to deforestation that made way for sugar and cocoa plantations. In their belief that they were racially and culturally superior, Europeans not only imposed their language and standards of living, but also abused and murdered people from native populations. European men raped women in Africa, Asia, the Caribbean, and Oceania with impunity.
However, your question asks us to play devil's advocate and consider imperialism from the point of view of those who sought, in Rudyard Kipling's words, to "take up the white man's burden"—that is, to provide "civilization" where Europeans perceived none. One must first ask, what did Europeans consider to be "civilization"? The definition may have varied, depending on the conquering nation.
France and Great Britain were Europe's most successful conquerors. Though Spain and Portugal were the world's first conquerors, their empires were less extensive and were not maintained for as long as those of France and Britain. The French began to seize islands in the Greater Antilles in the 1600s, during the reign of Louis XIV. Guadeloupe was among the first. The French maintained colonies in the Americas, North and West Africa, the Middle East, and Southeast Asia until the 1960s. In fact, they still have territories in the Caribbean, including Guadeloupe and Martinique. The British also began to colonize in the 1600s. English colonists established Jamestown, which was a failure, in 1607. The British maintained islands in the Caribbean, like the French, for its very lucrative sugar trade. Barbados was key for the cultivation and trade of sugar. The British also colonized Oceania and had a presence in Sub-Saharan Africa and the Middle East and were responsible for drawing the borders that later formed Iraq. However, India was Britain's most important colony. Due to the expense of maintaining India after the Second World War, the British quickly relinquished power in 1947.
The French believed that "less civilized" people would benefit from learning the French language, which was, and remains, a source of great national pride. France also introduced aspects of their national diet. For example, the Vietnamese sandwich banh mi is made with a baguette. The British built infrastructure such as roads and bridges. Though some might say that such an effect demonstrates that imperialism was a good thing, the British did not build such structures for the benefit of the locals; they did so to help the colonizers and businesspeople who had settled there to move goods and people more easily and quickly.
The colonizers brought other forms of technology and modernization as they became available, such as railroads, electricity, automobiles, plumbing in favor of well water, and airports. The French introduced their mode of architecture. A traveler who goes to Vietnam or Cambodia today will see numerous houses and buildings in a neoclassical or Haussmannian style, some in great condition and others in disrepair, built by the French in the nineteenth and early twentieth century.
Based on this information, it is for you to decide if the Europeans helped or hurt other people with imperialism, or if their actions were both helpful and hurtful. It is also worthwhile to consider the long-term economic benefits of imperialism in Europe versus the nearly indomitable poverty in many nations that were colonized.