I know this isn't a constructive comment but... Really? Like you don't see the point at all?
We should absolutely do that. In fact, we almost certainly will. This isn't mutually exclusive with sending humans, though, it's something we'd do as well. I think the primary function of said bacteria would be to help terraform the planet - warm up the surface, add gasses to the atmosphere, evolve new Martian life forms to continue those processes, etc.
We should, but there are 2 reasons we are not doings so right now.
1) We don't want to destroy our ability to determine if there was life on Mars in the past, which would be much harder to recognize as martian life if earth life was all around.
2) It is quite difficult to come up with something that could survive and reproduce on Mars.
I don't think (1) is a compelling enough reason and (2) is just another way of saying that it's hard, so I completely agree, we should do it.
Why not send bacteria which may evolve or may create environment for other form of life? If by known life you mean human, why is it so important to ensure the continued survival of human species in the universe?