Hell is for people who do bad things in a bad intentions. We hate such people. We hate cruel people, who hurt other people. Everyone have such people around them people you want them to die but they don't because you are not the one to decide who and how they die.
But if those people who hurt you just living happily, despite the injuries and the hurt you received from them, they still enjoying their life there, will you be happy?.
God given them a chance while they are still alive to show some regret and to ask for forgiveness, why not God just make them die or punish them at that moment for what they did? Because God is the justice and the most merciful, God give everyone time to realize their fault while they are alive and ask for forgiveness. Even the smallest regret is considered.
In Hell, not everyone is punished the same, every person takes what he only deserve, if he hurt a one person he will be punished only for that, if he killed a human he will be punished for that, he won't be punished for other people's mistakes. God is justice. Because you were hurt, that is why they will be punished for it unless you forgive them. If you don’t call it "just" then what you call it? There is a need for Hell indeed. And I'm not the one who decide who should be punished but God , because God knows better what inside those people and what their intention was while human can't know what other people hidden.