If Linda Franklin had been on some sort of target list, she would have had to have been pretty far down on it.
Hitting a target on a list is inherently more constraining, and thus more dangerous, than simply hitting a target of opportunity. You might draw attention compiling the list. Or you might get noticed tracking the targets (after all, your targets are unusually wise to surveillance techniques). But if you simply go after random individuals, then you are free to focus on seamless infiltration and exfiltration and minimize your chance of getting caught.
Also, you wouldn't want the public to get the idea that you are only after USG personnel. If they got that idea, then the ordinary citizens might not be terrorized. The aim in this case is to instill the belief that nobody is safe.
So, I conclude they simply got a little unexpected bonus with Mrs. Franklin.
Well, here's what occurred to me about that victim. The shooter might have been staking out the fbi building and just followed someone home, some likely employee chosen randomly, who looked like she was leaving work for the day, then he just stayed with her till he found his chance.
But I suppose the next question is, why didn't he just shoot her there leaving work?