"Where would be justice if god sent good people who aren't Christians to hell?"
Justice would be with the God of justice. To illustrate this, guess which famous American lamented the injustice he had suffered
with the words:
"I have spent the best years of my life giving people the
lighter pleasures, helping them have a good time, and all I get is abuse, the
existence of a hunted man."
It was Al Capone. We know Al Capone as a gangster and a murderer, but Al Capone,
who considered drugs and prostitution as "lighter pleasures" thought he was a
pretty good person, a man of justice.
Your standard of justice is most likely higher than Al Capone's, but God's is much
higher still. In fact, the Bible says that God's standard of
justice is absolute perfection and defines sin as anything short of it, including
seemingly innocuous things like jealousy (Exodus
20:17), lust (Matthew
5:27-28) or being angry without reason (Matthew
5:21-22). The Bible adds that everyone has sinned (Romans
3:22) and that the mandatory sentence for sin is the death penalty (Romans
6:23). That's the bad news.
The Gospel, which means "good news" is that God in His love decided not to wipe us
out. But neither could a God of justice simply let our sins slip by since turning a
blind eye to injustice wouldn't serve justice. So what did God do? He came and took our
death penalty upon himself.
If that is tricky to understand, imagine that you have been found guilty of a crime which carries a mandatory
death penalty. At your sentencing, you stand before the judge, who happens to be
your father. Because your father is a good judge who needs to uphold justice, he
confirms your penalty of death. But then, as you start to panic, your father
rises from the bench, takes off the judge's robe, walks down to you and tells you that because he loves you, he will take
upon himself your death penalty so that you can be spared, and he does.
The cross of Jesus (study John 18) was where the Justice of God collided with the love of God,
and Jesus was the only one capable of absorbing the force of that collision.
So, to answer your question, the Bible says that people who are "good" as defined by God go to
heaven (see Bible verses about
heaven), and the rest go to hell
unless we truly recognize ourselves as falling short of God's standard of
perfection (i.e., are "sinners") and believe:
"God so loved the world that He gave His only begotten
Son, that whoever believes in Him should not perish but have everlasting life." (John 3:16)