It's true that theatre critics CAN destroy actors' and theatre staffs' careers and lose people their livelihoods with their negative reviews, and so that's why he's going after the critics that constantly bashed him and is killing them in revenge.
It's such a good motive for revenge that I'm tempted to side with Lionheart, considering the shills we have as "critics" these days.
You do have a point here. However, I don't think that those critics had been bashing every artist. I'm pretty sure that, while they did destroy some of them, with needlessly cruel words, they also helped other actors get their careers and earn their bread and public respect. Their criteria may have been dubious, but ultimately Lionheart IS frustrated that he was not on the good side of the critics, unlike other actors, such as the dude who got the award...
The whole thing is a complicated messy business. Art does need criticism to survive, and the artists who make it are those not easily crushed by bad feedback. On the other hand they are also supposed to be sensitive creatures in order to produce good art.
My impression was that Lionheart was sensitive where he should have been gritty, and rough where finesse was requires (and now I'm waiting for Vincent Price to organize my elaborate murder, probably hinting to Sloth)(wait I'm not a famous critic; whew that was close).