When we emulate Hashem’s traits, we only emulate the “positive” ones — chesed and forgiveness and the like — and not the “negative” ones like punishment for sins or being a “jealous G-d.”
So if someone does something bad to us, that does not give us the right, under the principle of emulating divine traits and midda k’neged midda, to do something similar back to them.
However, certainly if someone does something kind to you then you should also do something kind for them. (But I suppose you should already be doing kind things for them anyway, since G-d is good to all.)
Tomer Devorah, by the Ramak, is available in English online for free. It’s a good manual for emulating Hashem’s traits.