"And 20+km underground on Mars it's not frozen any more... "
Can you tell me why this is the case? If volcanism is dead a few billion years, wouldn't it be pretty much cold at that depth? Do you think that the weight of those 20 km is enough to create that heat- even on a geologically dead planet?
I hope that is the case- we have found microbes deep under the surface of the earth. I have no strong opinion on if life formed on Mars... I DO strongly believe that if it did then life HAS to still be there- most likely buried deep, most probably VERY simple.
Short of turning the top few kilometers of the earth into magma, I dont see a way to wipe out all life on earth- If life formed on Mars I can't help but believe that it would be similarly hard to eradicate..