He cannot murder anyone unless he has permission. eg Job. If the Devil could do whatever he wanted he wouldn't be just looking for people to devour, he would be actually devouring them left, right and centre. You would have the Lord's authority as something meaningless and without power to control evil. And it's not based on one event, there are several instances where it shows demons are under the Lord's authority and cannot do whatever they want, whenever they want. That also stands true for men eg. Balaam.
The means of death was decreed, eating from the forbidden tree, iow disobedience. That disobedience had an effect on his whole person, spirit, soul and body in the day he ate. There is nothing to suggest Adam saw a day as a thousand years or that the effect of eating took time. The scripture makes it clear the effect was both immediate and over time exactly as the Lord said it would be, "on the day you eat from it, to die you shall be dying".
Go find yourself a decent teacher, one appointed by God and has prepared himself to do the work of God and learn what it means to be under authority because you seem to be misinformed about how authority works.