Europeans caused massive death in the Americas. I do not think we should replicate that model.

Also, the chance is small, but there might have been a separate biogenesis (beginning of life) on Mars. Sending humans with our dirty microbiome would almost certainly wipe any evidence of that, and possibly cause an extinction of an entirely separate form of life, which would be a crime even more horrible than the extinctions and genocides which we have caused so far.

Let’s just leave Mars alone until we’ve studies it more and are certain there is no life. Colonizing the moon seems challenging enough for a couple centuries…