Couple things....
-- Aresh is some creeper who wants to restart "biotic death camps" (thank you, Tela Vasir). Wanting to kill that guy is hardly indicative of being trouble, any more than, say, young Kaidan killing his abusive instructor (who happens to also be training/leading biotic kids).
-- Convicted criminals can still get jobs, and yes, even as teachers, if employers are truly desperate to fill open spots. Competent human biotics are rare commodities, what to say of those as powerful and experienced as Jack.
To that end, Jack is not an elementary-school teacher whose job requires nurturing very young children. She's more like an athletics coach or trainer, and for teenagers -- an age group with whom she would probably identify with and be well-received (can't tell you how many times I heard "Jack is just there for teenage players!" in the ME2 days). You don't need to be particularly smart, caring, patient, or anything you'd expect of traditional teachers to have the job she has, basically teaching gym class.
-- The kids in question are training for combat/war, so it makes a kind of sense to hire the killer biotic to train them.
-- Nobody would simply hand her this job. She'd likely interview for it, first. In the talks Shepard can have with her in ME2, Jack never struck me as a "bad" person, just young. Given that, I can see why someone who actually talks to Jack would feel like her criminal record is not indicative of who/what she really is.
-- Dare I bring up the events of her particular ME:Foundation comic? Jack works with children there, liberating them from Cerberus. Obviously, she saw herself in those kids and wanted to help spare them the same pain. Could it be that the explanation for her ME3 role is similar? Say what you will about the comic series, but I don't find that to be a stretch for her character, though I suppose others may judge her differently so YMMV.
I'll agree that the path they took for this particular decision minimizes the feeling of the player's choice making any real difference, and that expectation exists implicitly. I was expecting to see something where the Renegade resolution would lead Jack down a more "mercenary" path. However, the fact that she has a past (so to speak) alone is not really compelling argument against her current role, in my eyes.
*snip*
I think it'd have been interesting if Morinth showed up at the Ardat-Yakshi monastery, attempting to rescue her sisters (emails in ME3 which were blocked before they could reach Rila and Falere suggest that she wanted to do so). Morinth, of course, would want to free them for an entirely different purpose than Samara does - and if both of them are alive on account of failing Samara's loyalty mission, it could have resulted in an explosive encounter as their conflicting rescue missions collide.
That, I can agree with. Would have been really interesting to see Morinth's reunion with the sisters.
Seeing Morinth and Samara face off in the scenario you described would also be cool, but it would arguably be rewarding failure, so...