Dilbert acted out of self-preservation. If Alice had gone Nova, the damage would not be limited to just the CEO; Dilbert would have been collateral damage. So, to save himself, he had to shut her down.
Now, a better alternative might have been to put himself inside the containment device. Let her take out the CEO (and anyone else) while he was safely ensconced in his steel cocoon. Of course, there may be other people in the room, off-panel, in which case Dilbert acted to save them all.