That was supposedly an accident.
You are certainly free to suppose anything you wish, but I find no Biblical support at all for such a supposition. Rather, the Bible actually makes a plain, straight-forward case that it was premeditated murder. Please consider:
1. Cain had a strong motive for murder:
“In the course of time Cain brought some of the fruits of the soil as an offering to the LORD. But Abel brought fat portions from some of the firstborn of his flock. The LORD looked with favor on Abel and his offering, but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.” Genesis 4:3-5 (NIV) (and see 1 John 3:12, below)
2. God warned Cain against acting on his jealous anger:
“Then the LORD said to Cain, "Why are you angry? Why is your face downcast? 7 If you do what is right, will you not be accepted? But if you do not do what is right, sin is crouching at your door; it desires to have you, but you must master it." Gen. 4:6-7
3. Cain murdered Abel anyway:
“Now Cain said to his brother Abel, "Let's go out to the field." And while they were in the field, Cain attacked his brother Abel and killed him.” Gen. 4:8
4. God punished Cain for murdering Abel:
“ The LORD said, "What have you done? Listen! Your brother's blood cries out to me from the ground. Now you are under a curse and driven from the ground, which opened its mouth to receive your brother's blood from your hand. When you work the ground, it will no longer yield its crops for you. You will be a restless wanderer on the earth." Gen. 4:10-12
5. Finally, the Bible specifically calls it "murder":
"Do not be like Cain, who belonged to the evil one and murdered his brother. And why did he murder him? Because his own actions were evil and his brother's were righteous." 1 John 3:12