[lukan]

"Oh, if we can be absolutely sure that Mars has no life on it"

How can we ever be, if life might be hiding deep underground in vulcanic active areas?

That would mean stopping the advance of life for academic curiosity reasons.

For me science is mainly there, to help humans understand the universe to better find a place in it.

So yes, be gentle where possible, but a planet cannot be transformed in a gentle way. Life is rough where it spreads, it doesn't conserve things.