Pretty much every elected legislature has rules about what to do if some members of the legislature are unable to fulfil their duties, for whatever reason (illness, conflicting schedules, death, whatever). Most involve having a quorum of the elected body present, and everyone agrees that, so long as the quorum is met, the actions of the legislature are legal and binding until overturned by a further vote, or by some higher authority like the President or Supreme court.
So it’s not necessary to kill all the Democrats, and lots of the Republicans. You need to just kill enough of the Democrats to eliminate their majority in the legislature, while still maintaining sufficient numbers for a quorum.
Then, after the rabble are cleared out, the remaining members vote, with the majority now supporting refusing to certify the election results*, tossing the results to the House, or the Supreme Court, both of which are dominated by the very people who most want to overturn the election.
Bim bam boom, the election is overturned, Biden is out on his ass, Trump is still president, and it’s all perfectly legal, if you just ignore all that messy murder 'n shit, which the QOP will.
*“Oh, but that would never happen, they’d come to their senses!” Nope, even in the real world, we still had significant numbers of Republican members of Congress voting against certifying the results, after the vote was resumed after the mob had failed to kill anyone. If the mob had actually succeeded, even more of them would have voted this way, either because they wanted to, or were afraid not to.