Doesn't have to go too far, and probably shouldn't, into the realms of comically hated or vile, but just make him or her have some attitude or personality problems

Heh my favorite sci fi book series is The Gap Cycle where two of the three main characters are completely awful people and nobody is really "good".

My one line description of the series is X-Rated Star Wars (but harder sci fi) with a lot of rape, torture, and lovingly detailed graphic violence.