It will ultimately boil down to a question of morals: Is it morally right for humans to disturb the evolution of life on Mars for the sake of our own survival?
I'll not be the one to answer the question, but humanity is going to need to answer it very soon. Mars is the only other planet in this system that appears to have even a ghost of a chance of supporting a human colony. No other planet or moon even comes close. Discovering life (even advanced life) on Mars will not change the fact that Mars is our one and only hope for colonization.
The bottom line is that in order to ensure our survival we absolutely must leave Earth and colonize a second planet, then a second star system, and so on. We may have a few billion years before our star system will no longer support life as we know it, but the useful lifetime of our planet might well be far, far shorter. Given that it may take centuries to terraform Mars and establish a thriving human colony there, it is imperative that we begin as soon as we possibly can.
Thus, we might very soon need to decide whether it is ok to interfere with (and perhaps even destroy) another form of life for the sake of our own preservation. It is an age-old dilemma, and one that is now being posed to us on a greater scale than ever before in our history. It goes very deep - to the very heart of what life is, what free will is, and what it means to harm, interfere with, or destroy these things.
Science does not have an answer to any of these questions - at least no answer that is congruent with existing moral beliefs. These are not easy questions. Will we be the laughing stock of our galaxy for staying and dying on our home planet, or will our race be thrown in a galactic prison for being stubbornly arrogant and morally destitute. Who can know?