Tara was a psycho and plain and simply evil. No unfairness there.
As for Slade, from the get go, Wolfman made it clear that he was after the Titans to honor the contract accepted by his son, a contract he had no intrest in. However, he had that crazy honor thing going (not that having honor is crazy, but the logic behind his was, IMHO, a tiny bit insane). As soon as he fulfilled the contract, it was over for him (which made it hard to work him in as a recurring villain, than his sort of "redemption" and acceptance as an ally and anti-hero by the team).
As for the sex scene, it's ambiguous enough for anyone to think what they like about it. I do think they slept together, but I don't think Tara was used in any way (if anything, she was the user and instigator) and I can see Slade thinking the end justifys the means (although I'm sure it's an act that must have haunted him over the years).
Peace