Lol packing the court is categorically different than assassinating them.
I agree Democrats should at least be trying to do more (specifically with Obama and Garland), but it's not as simple as you're describing it. At minimum, what you're describing would lose a bunch of Democratic support, and if a bunch of your own party is against you it's ultimately not going to work. Now of course the fact that a bunch of Democrats would defect over this is itself a problem with the party, but that's the reality of the situation. There was no one weird trick that was guaranteed to work, and there are consequences to trying and failing.