Spreading foreign life that kills local life (even if by just out-competing on resources) sounds a bit like terrorism though.

But I have hard time believing even hardened organisms like moss or tardigrades could survive millions of years of hard vacuum and extreme cosmic radiation. Maybe embedded in some properly protective envelope, 1 out of billion trillion might. And then that one has 1 out of billion billion trillion chance to land eventually on a place that could be called livable. Or add few extra zeroes.

To kill local life, it first must exist, which is not confirmed at all. And if it exists, it is likely way better adopted to the local conditions.

In genetal, nature works with small chances, look how many seeds a plant gives and how few of them will be a new plant.

(Or how many sperms are created for 1 human)

But sure, chances here are way, way lower.