Can somebody remove their bad karma by believing that it doesn't exist?

If somebody does sin or bad karma, harms other living beings, but that person doesn't believe in karma and believes that karma is just an illusion. then how does his bad actions will harm him? For example, if I kill a mouse but I totally believe that nothing will happen, karma or religion is just people's belief, then will I ever be punished for that karma and such believe? Because there is also one saying that everything is just our belief? And also if I later ask for forgiveness to god for my bad karmas then will my bad karmas be forgiven?