Mars has stational liquid water on its surface, a CO2-rich atmosphere and its soil might lack some nutrients that plants need to grow there.
So, what is stopping us from filling a rocket with seeds and fertilizer and crashing it on the surface of Mars?
For one thing, I'd bet it will be way more interesting to see how life brought from Earth evolves in Mars that trying to figure out whether life eventually originated in Mars in the first place. Besides, it might never be proven beyond any reasonable doubt that there is/was life in Mars before we start terraforming the planet.
Also if someone, a government, company or individual, would start terraforming Mars on their own, what could be the implications? Are there any laws to prevent them doing that? What could the scientific community do to stop them?