I think that if there is something you can do to prevent or discourage the use of your creation for evil purposes, then you should do everything in your power to do so.

Sometimes doing so is also critical to keeping your users, especially if they entrust you with something useful but dangerous.

At Loopt I think about things like this a lot, and I hope that doing so has made the service safer and harder to subvert for common questionable or evil purposes.

Simply ignoring the issues would have been negligence.