User blog comment:Garhdo/Unhappy with the ending? Make your own!/@comment-1786741-20130225182538/@comment-3543731-20130227043211

Think of what the Catalyst does like pruning a plant. He believes, that highly advanced races are dangerous for the whole (all organics). By removing what he believes to be a harmful element to the galaxy (like cutting away withering leaves on a plant) he aims to save the rest of life from dying.

What he believes is that the technologies we create (mostly synthetics as they are harder to control) have a very high probability of wiping out ALL life in the galaxy. So, he gets rid of the ones creating those technologies, those that he believes are a danger, and allows the younger more ignorant races (the Raloi and Yahg for example) alone and, in his eyes, safe.