At first, the title of the post made me think that we killed all (possible) life on Mars, not just in the samples taken, just by having landed there and contaminated the planet. Now that would have been a true tragedy.
If there was life there 50 years ago there's good chances it has survived til today. Afaik there hasn't been any change in mars that would eradicate whatever life there is. We mightve killed the single organism we found but if we found one we can find more.