What was the policy of Mao in Tibet? Can we accuse him of being imperialist just like Stalin? I wanna make this issue clear 'cause i've heard lots of remarks accusing Mao of executing Tibetians unjustly...

Well it was during Mao Tibet was occupied. The reason for this is geopolitical one not an economic one.And Mao didn't murder millions of Tibetians because their population at those times was not much and about half of them fled to India and the Rest didn't oppose Chinese occupation at that time.

Prior to liberation by the People's Liberation Army, Tibet operated under what was essentially a feudal mode of production which entailed the ownership of land by a small group of landowners sanctioned by the theocratic state. As such, Mao's decision to liberate Tibet could not be described as 'imperialist', although there were certainly ways in which the decisions of the local government did not really reflect the interests or opinions of the peasants and proletarians of Tibet - such as the desecration of religious monuments that were of great value in Tibetan culture and history.

Any (Or rather, the vast majority of) Tibetans that were executed were reactionaries who tried to destroy the institution of Socialism in Tibet and thus the use of lethal force was fully justified.

It was an extension of his plan to unite all of China. Tibet had formally been a part of China but after the decline of the Qing dynasty it had separated.

Asides from the repression during the late 1950's as the result of a rebellion and the violence that occured during the Cultural Revolution, I haven't heard of many accounts of unjust executions.