This question cannot be answered.
I feel i could logically prove that this question cannot be answered.... but that's boring, and does not get us closer to the answers we seek.
First, what does "modern age" even mean?
Most people accept the propaganda that we are soooo advanced.
I mean, just look at us! We are able to confine thousands of criminals in tiny block buildings. We are soooo advanced!
In many senses, the modern age has gone backward. A police man could usually solve a crime because the list of suspects could be counted on one hand, and usually ended up as "Cooter, you know you are supposed to ask before borrowing something, now you got them all riled up..."
Also, before the modern age of long haul trucking, communities were very dependent on themselves. Thus they had a reason and a right to help others near them. The town idiot was used to deliver notes and sweep board walks. Today, the town idiot is told to get a house a car and a job, you lazy bum. Any of which is too hard for the fella. And so, we have been plagued with a homeless crowd ever since we created cities.
So, in modern society, we have outcasts whom no one cares about. I mean, soup kitchens and flop houses are hard work (emotionally). This means that the definition of "modern society" is a society that does not care for the bottom rung. And thus, the criminal code must be streamlined to process these people in and out of jail.
An eye for an eye, a tooth for a tooth
If you knew you would lose an eye if you poked out someone else's eye, you would be very careful not to damage someone's eye. Unless, of course, you have nothing to lose.
As a deterrent, this adage works quite well. (as long as it is actually enforced all around. Today, half of the murders are not even investigated.)
But the real piece here is justice.
True justice would be for the convict to restore the victim to the position they were in before the crime.
But, since we don't have reconstructive surgery like StarTrek yet, then for many crimes and accidents, there is actually no recompense.
So, we as a society have to figure a pattern that provides something back the victim.
In our "modern society" that is usually in the form of cash payment. However, the cash is a small comfort compared to most losses. Further, crimes are usually done by the bottom of society... which has no cash.
In the end, physical punishment is often the best.
Caning is seen as abhorrent here in the west, but it works quite well.
Its important attributes is that people, both the victim and possible perpetrators, get to see the punishment and the pain it entails. If you are going to have a deterrent, it needs to be nationally televised.
Now, we ask about the "Death Penalty"
Since i know reincarnation to be the way of the universe, my feelings on death, and thus the death penalty are quite different than most.
That said, does the death penalty do its job?
What is its job anyways?
Here is the typical example, in a fit of passion, or a blinding rage, someone killed someone else.
And, to bring justice, that person should die.
However, of all the crimes, murder is the least common to be repeated.
Unless they are a psychopath, the murderer has probably had the worst experience of their lives, and will be even less likely to murder in the future.
So, is it a deterrent? Not really.
If a murder trial takes years to be resolved with many appeals, starts and stops, where no actual truth is discussed, and then finally, years later, the murderer is electrocuted (with only a whimper in the press)... is it a deterrent? Not at all.
Next, that murder trial has to be completely played out to get to the death penalty. That court case costs more than keeping the person in prison until they die of natural causes. Financially speaking, that is just shtupid.
Therefor, in our "modern society" the death penalty is useless and costly.
It does not act as a deterrent, and any justice is lost.
The solution to this problem lies outside the rickety, rusted structure that to most people is seemingly set in stone... human nature.
In the future, we will have communities that care about all the people. There will plans and guidelines to make sure no one falls through the cracks. The way to avoid criminals is to avoid making criminals.
Currently, if we look at statistics, single motherhood is the leading cause of incarceration, suicide, drug addiction and about anything else you want to look at. In the future, single motherhood will be abhorrent to us. The child will be chosen to be born into a community, and the mother, and the father, will not be left to fend for themselves. It will be made sure that the child does not have added stress from the mother or father being stressed. Teaching of the child will be interactive and completely customized for the child. And by the time the child is 13, they will be a productive part of the community.
You don't steal things when you know how, and that you can get one yourself.
You don't steal things when you know the joy of creating it yourself.
No law could ever be made to mandate these things.
So, "the law" can only be used to clean up the mess after its happened.
And, right now, "the law" is doing a horrible job. (unless of course what you want is the destruction of society and the disenfranchisement of a large part of the population)
All other images in this post are my own original creations.