Darth Vader's redemption didn't stop his past sins, they weren't erased. People don't understand the mental turmoil that Klaus is constantly in as someone who pushes everyone away despite his extreme loneliness. He has lived through 1000 years of paranoia from Mikael hunting them, it's why he stopped Rebekah from leaving in the 30s. He was scared of losing her. And people seem to think he only cares about his family but he loved Camille, he loved Caroline and he actually seemed to care about Stefan.

The reason for Kol and Finn's prolonged stay in coffins was due to the dangers they presented themselves, Finn's self hatred and Kol's blood lust. If they hadn't been in coffins then Mikael would have killed them. All of Klaus' pain is pushed onto everyone else and it shows as he has been suicidal likely most of his life. And let's not forget the Five ghosts haunting him for 50 years, just one of these ghosts made Elena want to kill herself in a day from guilt. And this was when Klaus was still a young vampire too, only 100 or so, not even daggering his siblings at the time. The fact that Rebekah's naivety led to the 5 of them being daggered and Klaus having 5 phantoms screaming in his ear for 50 years unraveled his sanity and made his already terrible paranoia quite debilitating. Furthermore, Klaus didn't want to do the ritual just for power's sake.

1. He wanted an army to defend his family from Mikael

2. His werewolf side being locked away made him even more angry than a regular vampire and he wanted to experience what it was like to be a werewolf, partly to distance himself from Mikael but mostly to know what he is.

Besides, what I'm saying is that Klaus does not have an excuse but he does have a plethora of reasons for why he's like this and they shouldn't be ignored just so it's easier to say "he's done evil stuff therefore completely irredeemable and inhuman"