Biden supporter here, but I agree that the issue of looting and violence is of great importance. It pulls both ways though:
Some swing voters: the Democrats are way too soft on all this. Trump stands for law and order
Other swing voters: Trump is always pouring gas on the fire and just makes matters worse.
I hear both from people who are not hard core Democrats or Republicans.
Which way will be the greater pull, nobody knows yet. But I think this issue is actually the most decisive one right now, more so than Coronavirus, health care, China, the economy, all issues where most people know already where they stand and who they support.
The fact that this issue is at the top of the list may give an edge to Republicans, who are traditionally associated with law and order. We’ll see.