How did european leaders and citizens justify their imperial conquests and their seizure of other people's resources?

as justification usually served a religion ...especially in the case of European Catholic societies imperial conquest took place in the name of Catholic religions, spread of divine word and truth. Projects of expansion took place as missionary activity that was supposed to civilize (i.e. to westernize) "underdeveloped" savages/barbarians. It means that these missions usually contained contingent of priests or monks. This is typical for Iberian and French missions especially.

Explanation: Europeans usually thought in an Eurocentric way (something that continues even today but it is not so strong) and believed that the "divine truth" was revealed to them whereas "The Other" was savage, barbarian, underdeveloped and needs to be instructed how to live. But all the thing was not so one-sided because "The Other" usually followed Europeanization and considered it a criterion of a good life.

this isnt