Off the bat, I think it is important to clarify something.

I read somewhere that they cut off hands and stuff.

This happened, and it was infamously documented in photographs and in the Casement report.

Why were the Belgium colonists so cruel in the Belgium Congo

However, this happened during the era of the Congo Free State, circa 1885-1908. The Congo Free State was venture of the International Society of the Congo which pitched itself to western audiences as a philanthropic organization for the "civilizing" of the people of the Congo. Leopold II served as the chairman of the International African Association (a Nongovernmental Organization set up to explore the region) as well as the later International Society of the Congo which governed the Congo Free State. There were many connections between the International African Association and the International Society of the Congo, many personnel employed by the former would go on to serve administrative roles in the latter, even though the two were legally-distinct entities.

Anyhow, all of this is to say that the Congo Free State was not a colony of the Belgian state, it was essentially the private colony of Leopold II. Also, it was the violence of the late-19th and first decade of the 20th century that led to international outcry and forced the transition to becoming the Belgian Congo. So, it was believed at the time that giving the Belgian parliament oversight authority over the territory would get rid of the unaccountable practices that had occurred under Leopold II's personal reign.

Why were the Belgium colonists...

As a corollary to the above explanation, I think it is worthwhile to get into an explanation of the nationalities of the CFS administration, the Force Publique officers, and the rubber companies staff.

It is true that the largest portion of employees/staff of Congo Free State were Belgian. However, there were also employees and administrators who came from Britain, Netherlands, Germany, Canada and the United States. The explorer Henry Morton Stanley who famously explored the Congo basin for the International African Association was a British-American. Similarly, the military expedition to annex the Garenganze kingdom in Katanga to the Congo Free State was led by British-Canadian officer William Grant Stairs. Additionally, perhaps the most infamous of the concessionary companies was the Abir Congo Company which was established as Anglo-Belgium India Rubber company. The initial bi-national character of the company is apparent in the name, although British investors were later bought out and Abir became increasingly Belgian-dominated by the early 1900s.

All of this is to say that it is overly simplistic to characterize the European population of the CFS as "belgian" and to see the Congo Free State and the Belgian Congo as the same thing.

Now, the Belgian Congo period from 1908-1960 saw its own share of racism, resource extraction, and political manipulation of subject peoples. But, the massive atrocities like amputations and the deaths of 1/4 to 1/2 of the Congolese population happened under the earlier CFS regime.

Now, to answer the central premise of your question. /u/Khosikulu wrote an excellent answer about the background to an extremely famous photograph from the CFS era. I agree with his underlying answer, that amputations, raids, and other kinds of violence were a method of terror meted out by rubber companies and the Force Publique paramilitaries to frighten African populations into meeting rubber production demands. Or in other cases, it was the actions of sadistic men where brutality had become standard operating procedure; who behaved brutally in the belief that there would be no oversight and no punishment for their actions.