Because openly expressing your desire for Republican leaders to die will radicalize base Republicans, which will result in them either committing more terror attacks, or for them to even start demanding a newly elected Trump to be even more far right than he's already expected to be.
Regardless of your personal opinions, there was pretty much nothing else Bernie was going to say, other than perhaps nothing.
Nothing can be a very powerful thing to say. I wish he had said nothing.
Why can't he say "shame the shooter missed"?
Because openly expressing your desire for Republican leaders to die will radicalize base Republicans, which will result in them either committing more terror attacks, or for them to even start demanding a newly elected Trump to be even more far right than he's already expected to be.
Radicalising republicans is worth radicalising democrats. Republicans will radicalise themselves anyway, democrats need to be radicalised now.