What you’ve done is proposong a solution without clearly defining the problem. That makes your question sound appealing, it makes it sound rhetorical, but actually is meaningless without context.
Supposed the shooter had succeeded in his objective. One might imagine that Joe would win in November, maybe. But four years from now there would be a different candidate with just as bad views on those same issues, and the institutional problems that allowed Trump to gain power would still be in place.
Is gambling on 4 years of possible peace worth legitimatizing the policy of executing people whose political views you don’t like? That’s something you have to decide for yourself.
Many fundamental issues facing the country today go back decades. Pick your poison. Stacking the courts is an old strategy. Citizens United happened long ago. Redistricting and gerrymandering have been happening for decades if not centuries. All of those things matter, none of them were caused by Trump, and none of them would be fixed if Trump were gone. The systemic weaknesses can only be fixed by implementing systemic solutions, whatever those might be.