What actions are lawful?

The ones that are considered that way by the international community.

International law is designed in a way to prevent war and military/economic powers from exploiting others. So basically to uphold everyone’s right to autonomy and independence.

In this case the invasion would be extra unlawful because of bilateral treaties.

Am I having a positive impact on the world by simply acting against every country that does something I consider unlawful?

I mean usually you do unless you are against others having their right to act independently and keeping their autonomy.

I think most would consider that to be positive.

If I do so more to some countries than others, am I not acting in favor of some countries?

Yeah, you would very clearly favor the countries that do not invade others in this case.

I could go on and on but the answer to all of these questions is very clear to the absolute majority of people imo.

You need to perform some real mental gymnastics to arrive at a conclusion favoring an invasion if you based your reasoning on these questions and the respective answers.