Why were some people critical of Britain's imperialist practices?

Britain's imperialist practices were often only great for Britain, not for the colonies, which was seen in the US, India, China, and many many other colonies which were on the verge of disaster because of imperialist ruling.

I believe the answer is: Because colonizing often involved

In the past, in order to obtain cheaper workers and resources, Britain imperialist tend to colonoize a weaker country and force them to follow an agreement that provided the imperialist with what they want. Because of this, many people are critical on how they accumulate their wealth.