Prov 23:7 reads "For as he thinketh in his heart. so is he:..."KJV and in Matthew 15:18-19 we read, "But those things which proceed out of the mouth come forth from the heart; and they defile the man. For our of the heart proceed evil thoughts, murders, adulteries, fornication, thefts, false witness, blasphemies."KJV
In Genesis 4, after the Fall of Man, when sin had entered the Garden, we have the record of two brothers, Cain and Abel, who had a falling out. Tragically, their differences led to murder. Cain killed Abel; at first glance it seems that this act took place because Cain had a difference with Abel about worship.
Cain was angry because his offering was rejected by God and Abel's was accepted. Evidently, Adam and Eve had instructed their sons regarding proper offerings and for years all had gone well. That is, until the time came when Cain brought a bloodless offering, which was unacceptable by God. Cain grew angry at God...(Gen 4:5-6)...then took his anger out on his brother (vs 8).
Cain was angry, not so much because of what his brother had done, but because of what Cain himself had not done, in that he failed to offer the acceptable sacrifice. Cain was angry and when God confronted him and counseled him on how to make things right, Cain took his anger out on his brother...and slew him. Cain was ruled by sin in his heart and his anger which was directed towards God (Whom he could not attack) was vented towards his brother. Are you getting the picture here?
Sin in the heart was the source of Cain's behavior. It was because Cain was not right in his relationship with God that precipitated murder. In Matthew 5, Jesus warns that sin begins in the heart and is then manifested by the act of the hand.
We do not know what sort of weapon Cain used, but we do know that the source of the problem was in in his heart that precipitated from a wrong relationship with his God.
While many would not allow the thoughts and intents of their hearts to go so far, there are obviously those who do, and that is tragic. There is nothing that changes the heart like the Word of God...and oh!, how it is needed!
Ps 119:165 tells us that "Great peace have they that love thy law; and nothing shall offend them." KJV
Ps 119: 11 says "Thy word have I hid in mine heart, that I might not sin against thee."KJV
Perhaps if we were more concerned about what's in our hearts (and our children's hearts) than we are about what's in our wallets, our society would begin to be different.