@Sun your opening sentence to this thread is (I consider myself a sinner on this earth. ), I say for my self that I am the greatest sinner on this planet earth.
I cannot say about others but I have committed numerous sins which should not be pardoned. But as it is said that God loves your 70 times more than your parents, He can pardon you if you apologise.
Killing any living being is the most heinous crime on this earth. If we cannot give life to them then who has given us the right to kill them.
I remember a story here which might reply to your query. There was a person who was being tried for murder which he did not commit but all the evidence were against him. The judge was confused, but from the core of his heart, he was convinced that the accused is not a criminal. On the final day of judgment, the judge said, "I will pronounce the final judgment tomorrow".
The next day judge gave his judgment, "Hang till death!" The accused requested for a personal meeting with the judge and the permission was granted. The accused said, "I have not committed this murder". The judge said, "Yes, not this one, but you have killed ants in your childhood days and told an incident of accused's childhood days to him". The accused said, "Yes, he remembers that incident and the same has been haunting him since the day he committed it". The judge said, "Did you ever asked for an apology for the same anytime during your lifetime?" The accused said, "Never!" Finally, the judge said, "You are not being punished for this murder but consider that you are being punished for what you did with those ants in your childhood!" The accused was convinced.
We never know what sin we commit but we should always ask for His mercy and pardon us for our wrongdoings.
World without God is Zero without One!