I tried to explain that although that is true, a gun certainly makes killing easier.
His message also referred to how Chicago, Detroit, Washington DC, St Louis, Baltimore and New Orleans all have Democratic mayors. If all of these cities disappeared, the U.S. would fall from 3rd in the world in murders to the 4th least.
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Although specific cities may have lots of gun control, it isn't very effective. Baltimore (which of the list below seemed to have the highest murder-rate in America) may have lots of gun control. But Maryland in general does not. It bans short-barreled rifles or shotguns. Not a very long list, and sneaking weapons from outside Baltimore into the city isn't particularly difficult. Baltimore doesn't have border checks as far as I know.
Perhaps the mayor of Baltimore's intention was to make guns illegal so their price becomes higher on the black market. The higher the price, the less likely people are going to purchase. This is the same strategy used for illegal drugs. Limit supply as much as possible, people can no longer afford heroin and cocaine and purchases should go down.
Limiting supply for guns and drugs failed for different reasons. For guns, gun control nationwide is not uniform. A gun illegal in your metropolis may be easily purchased outside the city. A gun illegal in your state may be easily purchased on one bordering. Limiting supply in specific regions has little effect, as they can still be bought cheaply in a neighboring region for very little, with some small cost of smuggling in added on.
Drugs laws are actually fairly uniform across America (though with the legalization of weed in specific states that is starting to change). The strategy of limiting supply did indeed cause the price of drugs such as cocaine, heroin and meth to skyrocket (though not weed, because of lax enforcement and ease of access).
The strategy still failed for drugs because many drug addicts still needed the drug, and just turned to crime to get the cash needed to get their fix. In economics drugs are considered an "inelastic product" as changes in price don't actually have much affect on demand. It also made the profit margin so high for drug dealers that globally the U.S. is a lucrative market as people are willing to pay so much for drugs.
But if there was uniform gun control across the U.S. it is more likely to have the desired effect compared to drugs. Guns are not addictive. The bushmaster gun used at Sandy Hook costs $1000. You can purchase it in some states at Walmart. It is banned uniformly in Australia, causing its price to move up to $34,000. Very few sane criminals are willing to pay so much for a weapon, and would rather take the risk of doing crime unarmed or with knives (and stabbings have much higher survivor rates). The black market is also very restricted, and someone who is mentally ill is much less likely to make a purchase on the black market successfully.
The only people who are willing to make such purchases and have the mental capacity to do so are potential terrorists and organized crime. Both can only truly be stopped by a combination of surveillance measures and effective policing.
The vast majority of gun owners do not commit crimes. But "society only moves as fast as the slowest person." We have speed limits, demand seat belts to be worn, and ban DUI because a minority of people will make these actions without penalizations.
The 2nd Amendment was made in the 19th century so that America could have an armed populace to defend against foreign invasion. Today it is used as justification for people to own tools used in warzones.
