Make sure that you don’t abuse people; you don’t oppress people to be saved from the hellfire. If you committed a terrible sin, major sin but you seek forgiveness from Allah, he’ll wipe it out. Why? it’s between you and Him.
But the minute you slandered someone there are three people involved meaning three parties involved who, you and Allah and there is another guy or another person. Allah is Ghafoor, Rahim, most merciful, most forgiving that person is not most forgiving most merciful.
By right if you want to be forgiven for something you did wrong against someone else, you are going to have to put put your ego away, your pride needs to go aside; say, listen my brother I’m really sorry for what I said about you. I was totally wrong, totally unjustified and that’s it, please forgive me.