Allah says in the Quran the one who has attained ultimate success is one who enters jannah. There are 10 ways found in the Quran and sunnah to help us avoid Hellfire. The first of these is taubah – to return – to come back to Allah after sinning. No sin can withstand taubah. Allah will forgive all sins if we just turn to Him.
The fact that Allah accepts our forgiveness is what shows Allah mercy. The angels cannot sin so Allah cannot to show His mercy to them. He (swt) showers us humans with His mercy!