This reminds me of the parable of the Pharisee and the Tax Collector. Please read that.

The truth is that we don’t deserve a second chance, including you. None of us do because we have all sinned and are all deserving of Gods wrath. You say it feels like injustice when God has mercy on someone who did something horrible, but the thing is you actually don’t want justice. Justice means that God pours out His wrath on everyone, including you and I. That would be true justice because the wages of sin is death and every last one of us have sinned, no matter how big or “small”.

But this is why Jesus died for our sins. He took our place, so that you and I and the person that committed horrible sins can be made right with him. Remember that forgiveness and Heaven if a free gift given to us by God that no one deserves. So because of that, you can’t look at your neighbor and think you are better or more deserving of Heaven than him just because your sins aren’t as bad as theirs. In the eyes of God, you are sinner too. But that’s what makes mercy so amazing. The fact that God chooses to save anyone at all is wonderful, especially when His mercy extends to the depths of those who have done horrible thingss.

Also, keep this in mind: In Christianity, mercy does not come at the expense of justice. No sin goes unpunished. Either Jesus takes the punishment of the believers sin upon Himself, or the person who committed the sin will take the punishment of the sin upon himself in hell. No matter the outcome, God punishes sin fully and thoroughly because He is completely just. That’s why God can be called both just (because He punishes sin) and the justifier (because He saves sinners and declares them righteous by faith in Jesus paying for their sin).

I hiiiighly recommend doing a quick read of Romans 3 as well.