Why the God of OT is harsh
You have heard that the God of the Old Testament has no mercy and loves to kill people, and that every little mistake results in death. Though the appearance is true, the motives and reasons are a different story all together.
Let's compare the two different times for a moment: The Old Testament and the New Testament. In the OT, we see a lot of sacrificial deaths, killings, murders, etc., even in the YHWH worshipping Israelites, and we also see that too in the neighboring, pagan cultures. But when we came to the NT, when God stopped killing, the pagans also seemed to stop, or reduce their actions.
What we're seeing here is that when God does something for good, satan will create a very close counterfeit and popularize it so much so that one could not tell the difference apart.
Here is my controversial thought: Satan invented child sacrifices, ritual prostitution, and many other death threats to counterfeit God's sacrificial love for his people.
There were so many pagan cultures who kill for sport, so when God killed someone for an "insignificant" sin, we only focused on the fact that God killed and not on the why.
God's nature could be seen most clearly in the story of Abraham when he was ready to sacrifice his son Isaac. God may ask you to kill, even for a test, but at the end, He would love to see everyone alive and saved, that's why he provided the ram as a substitution - showing that God would rather let himself be killed as a sacrifice rather than having any of his precious children die.
God is love, through and through, but Satan made it look as if God killed out of spite and judges harshly. What we don't know is that the people God killed are like grown, baby Hitlers. They are so evil that there is no hope whatsoever for them to repent, but at the same time they haven't done much damage for the Israelites so God struck them down to protect his people.
Disobedience to the law of God is a sin, and it has very grave consequences. So whether that act be eating a simple apple, or something worse, it still has its consequences.