Why would god kill a man for not resting?

God gave very specific instructions as to how the children of Israel were to live their lives. This man and others would choose to rebel and disregard God's leading and instruction and place what they wanted to be and do ahead of how God set things up. They knew better and chose to ignore what God wanted and followed their desires instead. God looks on the attitude, the intent of our choices. God should be first in our minds and hearts, and our actions need to reflect this.