indigenous life on Mars wouldn't likely we completely destroyed by a few manned missions to Mars. If it has survived and thrived in that harsh environment for eons, then I doubt that we would be capable of destroying it.

It may be that we would introduce Earth flora an fauna to Mars that would have the potential to alter the long term evolution of Mars species, but why would that be in any way immoral?

We alter the genetic future of different species all the time through cross-breading, gene splicing, etc... and morality only plays a role in a few cases (and even in those the moralist fears are ofter grossly exagerated and overblown).

The only thing that would likely endanger native Mars based organisms would be if we did completely alter eMartian environment through terreforming. But even then, The native Mars organisms would likely adapt and evolve to the new Martian environment (and they would probably be much better at it than Earth based organisms).

Wheter there is life on Mars or not, we must eventually send humans, if for no other reason than to study it before it gets wiped out by some unforseen event at humans canot even control.

Scott Evil