Why are border cities like Minneapolis and Seattle so violent? Is it safe to visit Canada?

It's safe, but you should still be aware of your surroundings. There are thieves in Canada too. But the boarder is closed to non essential travel and it has been for about a year, you might be able to fly there. Also due to covid, there's probably not too many activities going on and check their covid 19 protocols before you go. 

Why are border cities like Minneapolis and Seattle so violent?

Many possible reasons most having nothing to do with being close to Canada.

Is it safe to visit Canada?

MOST that visit other countries have little problem. Crime does exist. Tourists should be careful and aware of their surroundings and practise basic street smarts.

Things like not wandering around staggering drunk in the middle of the night. Insulting the locals with YOUR political views.

The border with Canada has been closed for over a year and is expected to be closed for many more months.

Tourism IS NOT ESSENTIAL travel.

Crime rates are generally lower than in similar sized cities and towns of the USA.

You will not be allowed to enter Canada, even if you have tested negative for COVID-19 or have received a COVID-19 vaccine.The Canada-U.S. temporary border restriction implemented on March 21, 2020 continues. All discretionary/optional travel remains prohibited.

Before Covid MILLIONS crossed into Canada every year. Few had issues with crime.

Minneapolis and Seattle are in the US.

Canada is generally much safer than the USA.  Even in places close to the border.

Canada is generally safe, much fewer guns and angry people.