I think the Left is worried, and rightfully so, that Bolton will get on the stand at their urging, then when asked pointed questions he can answer truthfully in ways that, even if he doesn’t want to answer them, won’t be able to provide the damaging information the Leftists want to get out of him.
He will be a big, fat zero for them, even if he does hold a grudge against President Trump.
Adam Schiff has already demonstrated a clear pattern of eliciting and publicizing selective testimony from witnesses. If a witness testified before his committee but provided no evidence to support Schiff's allegations against President Trump (or even worse, provides EXCULPATORY testimony), that witness simply vanishes and is never heard from again. This is exactly what happened with the original "whistleblower" despite Schiff's previous statements that the whistleblower WOULD testify in public. It's also what happened with the close-door testimony of ICIG Michael Atkinson. This is why Atkinson should be Witness #1 for the defense.
F#%& Schiff, and f#%& his demands for witnesses and documents. His demands for this crap should be summarily dismissed until he turns over everything he already has.