It is a dead rock.

1- Probably but not certainly, and a little caution (say, 100 more years of remote sensing exploration as technology improves) will go a long way.

2- I'm not so anthropocentric as to think that the only value in the universe is human value. It's not our universe; we're an infinitesimal fraction of it. Other life has inhabited earth for billions of years, and maybe other places too. The terrestrial or extraterrestrial environments have value for existing and future life, regardless of whether that is human life. And most of life isn't human, and won't be in the future either.

There is no sentient life in Mars. If there is microbial life on Mars, it is not making efficient use of its resources. I will not trade the billions of future people that might call a terraformed Mars home for some scum on the bottom of a rock.

I don't necessarily disagree, but I do wonder if life exists somewhere in the Universe that would view humanity in the same way.

No doubt. We'd best be ready. Thankfully astronomical data makes this unlikely at least in our stellar neighborhood, so we have some time to grow in both power and maturity.

We have a small chance of exploiting 1% of Mars in less than 100 years. The sooner we get our first factory there, the sooner we'll know if there is any reason not to build the millionth factory.

afraid we are going to mess up a lifeless rock

It doesn't need to be alive to be beautiful. There is great peace and beauty in empty places. Mars has grand and majestic vistas: canyons and plains, dunes and mountains. Reducing it in your mind to a lifeless rock obscures the awe and wonder. It is the opposite of appreciation, it makes the idea small in you, worthless and not deserving of respect.

That is your perspective that it is beautiful. What if my perspective is that it is more beautiful inhabited, alive, and sustaining an ecosystem it probably hasn't for millions of years?

Beauty is really subjective.

You three sound like the argument between Sax Russell and Ann Clayborne in the Mars trilogy. If you haven't read them, I recommend them.

There are infinite similarly dead rocks in the universe.

Similar [dead rocks], or [similarly dead] rocks ?

Also, I think there are only finitely many eventually observable planets in the universe?

All of that requires an observer there to appreciate it. Messing up the scenery of Mars with industry is a champagne problem.

"Ruining a mountain side" is one thing when it sustains an ecosystem of life. It's harder to argue we are ruining something like a planet with no life.