Is it ethical to send Earth lifeforms to Mars?
Aside from the many dangers Earthlings may face in space travel, there is also the issue of contamination: we do not want to adversely contaminate Mars with Earth life (forward contamination), and for any possible Martian life to adversely contaminate Earth (backward contamination).
I say "adversely" because we want to ensure the life of one planet to cause excessive harm to the ecosystem of another planet (i.e. Earth bacteria can multiply on Mars and possibly compete with and destroy Martian life, and vice-versa.)