I suggest you read more history. Sudden violent events have, time after time, been used to increase authoritarian control. I can absolutely see a line that starts with "we need to get the Senate safe, also we're going to give emergency powers to the president." Followed by attempts to prevent the Senate from meeting, increasing police and national guard presence. Protests start nationwide. Protests lead to riots and conflict. Suddenly there's an incentive to use a little force to get everything "in order". And maybe just hold on a bit before handing over power.
> we need to get the Senate safe, also we're going to give emergency powers to the president.
There is an actor in your sentence that doesn’t exist in the US government. There is no “we” that would give emergency powers to the president against the will of the congress.
What if the violence ends in everyone who opposes the President losing their lives? They get rid of all Democrats in Congress, and Republicans vote to give Trump emergency powers.
Not true at all. First and foremost, "the whole number of the House has long been viewed as the number of Members elected, sworn, and living". So if they murdered all of the Democrats the remaining Republicans would have enough members to maintain a quorum.
All Dems murdered would be catastrophic circumstances and would require the 72 hour window with the failure to reach quorum report. That’s plenty of time for states to re-appoint all of the senators and prevent the take over of the upper house.
Who is the "we" that is giving him more powers? Trump is viscerally hated by the Speaker (2nd in line) and most of the House (they already impeached him once) and a significant portion of the Senate. No way he's getting any emergency powers, if the Constitution would even allow it.
For a more competent autocrat-hopeful, the military. Trump's biggest mistake (and our biggest boon in such a situation) is that he spent 4 years making enemies of the top brass. You don't become a dictator when the heads of the military hate you.
Especially because the police support Trump.