@aral yes, but killing Columbus would be still different story, than killing Hernando Cortéz and other conquistadors.

Not all colonialism was equal: there were many different strategies, some of it was mostly trade and left the originals cultures almost intact, and perhaps the benefit was mutual (at least some of the trade, eg. spice trading with Southeast Asia, could be considered as such case).

The spread of Christianity was sometimes much worse then trade and sometimes relatively innocent. And not all trade was slave trade. And not all colonialism was white Christian colonialism... I believe Arab expansion is more or less comparable in scale to European expansion. There was no basic difference from Europeans, when Turks conquered Balkans or Moguls conquered India. China was expanding very slowly, but still: the Han ethnic dominance on China mainland was simply result of slow conquest. Not only colonialism, but also assimilation.

The exploration urge is natural, the big question was "what next"? Trade and cultural exchange seems fair, slavery and conquest definitely not.