Overthinking the motive here. It's simple: abysmal self-esteem + victim complex = seething rage.
Fucker's spent his whole life feeling inadequate and persecuted, exactly like VT's Charlie Chan, that Eliot retard in Santa Barbara, and countless other 'postal'-types. Can't feel good about himself, so he starts to blame the world.
He chose her simply because 1) she replaced him, and 2) it'd air live.