According to the Bible, God created darkness, evil, calamity, and disasters (
Isaiah 45:7). If God actually exists, then he is ultimately responsible for the messed up world we live in, because he's the creator, who is omnipotent (
Psalm 147:5;
Job 42:2;
Daniel 2:21), omniscient (
Psalms 139:1-6;
Isaiah 46:9-10;
1 John 3:20), and omnipresent (
Psalms 139:7-10;
Isaiah 40:12;
Colossians 1:17). If God is, in fact, all-knowing, then he knew that Adam and Eve would both disobey him before he created them. Having infinite knowledge, he also knew that he would curse humanity with sin, punish humanity for sinning against him despite the fact that he allowed sin in the first place, and he'd sacrifice his son to remedy the curse of sin that he could have prevented before he created Adam and Eve, or he could have ended sin and rebellion against him with Adam and Eve rather than callously inflict humanity with a sinful nature.
If God exists, and he has infinite power, then he had the ultimate power to stop the Holocaust and save the lives of six million Jews and five million non-Jews. Furthermore, if God has infinite knowledge, and he is ever-present, then he had foreknowledge that the Holocaust would happen, and yet he did absolutely nothing to stop it. That's not a loving and merciful god. God had foreknowledge of every tragedy and atrocities throughout human history, and yet, he allowed all these horrific tragedies to happen, and he didn't stop any of it. Personally, I think he's cruel and sadistic. No doubt his followers will attempt to defend him, but there's no excuse.