What are the sins that God kills for in the Bible?

Stopping to think about why God wiped out the entire human population bar eight people, provides an interesting answer as to what sins God kills people for. Then, if we go a little further back in Genesis, we get the fundamental reason that covers all contingencies. Note, please, that this does not touch on which individuals have been struck down by God, in death. At the end of my answer I will give a simple reason why focusing on that can be dangerously misleading for us.

What brought the global flood that wiped out the world that was then, is stated in Genesis 6:5-8:

"The Lord saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time... But Noah found grace in the eyes of the Lord."

Note, please, that Jesus said that, "Just as it was in the days of Noah, so it will be" again prior to his sudden coming to usher in the Day of Resurrection and Judgment (Matthew 24:36-44).

Such times of judgment arise because humanity has become so wicked in their thoughts, and their desires to do evil work out in gross deeds, to such an extent, that God kills all unrepentant sinners who do not find grace in his eyes.

But I said I would go further back in Genesis to show the fundamental reason that covers all contingencies. That is in chapter 2, where God provided a Tree of Life. Sadly, the first human couple never got to eat its 'fruit' (and so gain immortality) because they decided to try another 'fruit' which God had warned them would cause them to die, should they partake of it. Their disobedience brought sin and death to themselves, and all their offspring. Yet, in the last book of the Bible, the Tree of Life re-appears in the paradise of God. By then, God has dealt with all sin, and all sinners. Repentant ones who have faith in his provision for forgiving sin will discover 'paradise restored', while all others will not. The former live forever in God's presence, in bliss.

Once we grasp, and believe, God's statements, e.g. "the soul who sins shall die" (Ezekiel 18:20; Mat.16:25; John 9:2-5), and "the wages of sin is death" (Romans 6:23) etc, we might stop looking at other people so stricken, and their particular sins, because the danger in doing that is to think, "Well, I'm not a murder [or rapist, or thief, or kidnapper etc]". Then we start to feel less anxious about ourselves, supposing that if we are (by comparison) better people than them, God should be quite pleased with us. Category mistake, there.

The named individuals in the Bible, and their particular sins, are there as a warning. We should learn from that and build up an ever-growing picture of the range of sins humanity displays. That goes from murder (yet God did not strike down dead every murderer) to liars (such as Ananias and Sapphira, but not all liars have been so stuck down.) Jesus teaches that even our thoughts can be so sinful that we're judged for them. It also flags up our reprehensibility for sins of omission (not just commission). This means nobody has a leg to stand on, before God, and a Day of Judgment is coming, when everything hidden will be revealed, and God will be shown to be utterly just and righteous. How we need to see sin from God's point of view, and not our own! And that includes the God-given hope:

"But where sin increased, grace increased all the more, so that, just as sin reigned in death, so also grace might reign through righteousness to bring eternal life through Jesus Christ our Lord." Romans 5:20-21 (NIV) Emphasis mine.

I've made a list of instances where God killed people. I hope this helps answer your question.

• Noah's flood. Gen 6-9
• Er, who was wicked in the sight of the LORD. Gen 38:7
• Onan, who displeased the LORD. Gen 38:9,10
• Sodom and Gomorrah. Gen 19:24
• Aaron's sons Nadab & Abihu, who offered strange fire before the LORD Lev 10:1
• The people that complained. Num 11:1-3
• The people that lusted Num 11:31-35
• Korah, Dathan & Abiram whom the earth swallowed Num 16:1-32
• People who murmured against Moses and against Aaron Num 16:41-49
• Discouraged people who spake against Moses and God. Num 21:4-9
• Some Philistines 1 Sam 5
• People who looked into the ark of the LORD 1 Sam 6:19
• Uzzah who put forth his hand to the ark. 2 Sam 6:6,7
• Death of a disobedient prophet. 2 Kings 13:11-26
• Ahaziah who wanted to enquire of Baalzebub the god of Ekron. 2 Kings 1:16
• The 2 captains of 50 and their men sent to Elijah. 2 Kings 1:9-12
• The little children who mocked Elisha. 2 Kings 2:23,24
• Gehazi is punished for his greed. 2 Kings 5:25-27
• Ananias and Sapphira who lied to the Holy Ghost. Acts 1:1-10
• Herod, because he gave not God the glory. Acts 12:23