“For if by one man’s offence death reigned by one; much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)”  
Romans 5:17

If the main verse had stopped at “For if by one man’s offence death reigned by one…”, we would have known that death “reigned”, and as we had established, the idea of anything reigning suggests that whatever is being reigned over must have a tendency to be rebellious or fight back.
If death “reigned”, it suggests people have a tendency to fight back, in simple terms- many people don’t want to die, but since death is reigning, they would be subdued to death and all it accompanying sidekicks- sickness, poverty, despair etc. against their will.

Glory to God, the verse continues;

“…much more they which receive abundance of grace and of the gift of righteousness shall reign in life by one, Jesus Christ.)” 
He is saying that this invisible death that subdued people unto its bidding doesn’t have the highest authority, there are “they” which reign MUCH MORE! Their reign is so vastly higher than that of death, that he pronounces the reign of death as the past: “death REIGNED”.
So there are people, that if death shows up and a man who has no plans to die or even get sick begins to sink into an early grave, if they show up have the authority to tell death to get out and it will obey against its will!
Let’s move on from subduing puppies to subduing death, because we can. We are the ones who have received abundance of grace and the gift of righteousness, we are the ones reigning now!
Remember, that we reign, means the subject have the tendency to be rebellious, so death might still show up knocking at your door with a pain or a lump in your body or whatever, but subdue it- tell it to get out and stand your ground as the king that you are!