The least harmful way to leave is to be upfront about it and tell her you are leaving. That is also the healthy, mature, way to leave.

There is not going to be a 'peaceful' or 'amicable' way to leave. It is going to be painful, either way. Running away behind her back is, frankly, the worst thing you could do to another human being.

Take your share of the responsibility in this relationship and give her a chance to hear a 'goodbye' and some sense of closure in knowing 'I am leaving'. Don't sneak out like a thief in the night, unless you want to create more damage than you could imagine.