Same thing that causes any crime: when one person prioritizes their desires above the autonomy and lives of others. And if you can sort that one out, you'll have solved humanity.
Then why are there so many shootings in the US, where gun laws are extremely liberal, and almost no shootings in parts of the world where gun laws are extremely strict?
Very few countries have the population we do, so a per-capita comparison is the only way to get close numbers. In most of the research I've seen over the years the United States is always down the list a ways in violent crimes when fairly compared with other nations.
A country with more guns will have more gun crime, that's usually true, but the threat of being stopped by an armed mark or bystander deters crime in general.
Look at the statistics on gun ownership and gun violence and see that the US has a very low rate of gun violence compared to a lot of countries in the world but it sure is reported on and promoted by people use that as a cause to control things
That's becuase other countries have low gun ownership, meaning that when one but job gets a hold of a gun, it skews the statistics. If 100 people own guns and 27 go nuts, and 5 people own guns and 1 goes nuts, the percentages are going to look a lot worse for country no.2, despite the fact country no.1 has more gun violence.
-driver error 0%
-other road user error 0%
-faulty car 0%
-excessive speeds 0%
-hazardous weather 0%
-Crashing 100%
The maths MADE SENSE AT THE TIME, lol