What's all this talk about obligation? Of course there's no obligation, and I mean that in terms of an aikidoist's obligation OR a human's obligation.
The real question is whether or not you are interested in helping the world out of desire for others to be happy--that's not obligation, that's honestly wanting others to not get hurt, and wanting to do something about it.
In other words, yes I think that as a person who tries to follow the way of aikido, to escape an attack and leave the guy loose to attack another is not the way to go.
(Then again we have a system to work with and instead of brutally going against it by hurting the attacker to discourage him, Kat is right that the police are the ones who are supposed to discourage him.)
I think Jeff's father is right and my answer to his question would be "just because I desire to dissapate the conflict without harming the agressor doesn't mean I'll let him loose afterwards.."