I don't know Shannon Watts from anyone

But this doesn't seem unique to her.  Nor this.  Nor this.  This is standard Leftist boilerplate.  The Left hates the Christian West and the American heritage.  It has deemed it entirely evil and racist and imperialist and genocidal and all that jazz.  In the 1960s the Democrats dropped Blacks and added WASPs to the 'group to fear', proclaiming that you can always tell a racist by the color of his or her skin.

The Left thus can accuse America of being 100% evil and 100% racist.  It can, based on ethnicity and nationality, declare all Founding Fathers 100% evil and 100% racist.  It can accuse all subsequent actions and defenders of those Founding Fathers as 100% evil and 100% racist. And it reserves the right to assume anyone who fails to conform to the Left is 100% evil and 100% racist.

Of course in the real world, a stronger case probably could be made that gun control has more roots in racism than gun rights.  But what would be wading into a debate based on facts, data, research, discussion and rational thought and might actually show one or the other to be true.  The Left is content to proclaim a dogma and declare all non-believers to be 100% evil and 100% racist.  In that, I see nothing particularly unique in Ms. Watts' tweets.