Furthermore, countries cannot just "get rid of" every bad leader of some other country here and there around the world just because they don't like him. You have to have much more pressing reasons than that to go to war. If it was okay to just get rid of "bad" leaders in other countries, then every leader of every nation in the world would be in constant possible danger of his country being invaded or himself being assassinated by foreign agents...all on the sayso of someone in some other country who decided he was "bad". That would be a complete end to international law in the world. You might as well just make the world totally lawless and rule it by brute force.

And it would apply equally well to getting rid of presidents of the USA then, wouldn't it?

So the simple rule is: don't do it to others if you don't want someone to do it someday to you.

The "good guys" don't get to break international law just because they think they are the "good guys". Hitler thought he was, after all, therefore he could do what he wanted, and you see what happened to him in the end. The same can happen to others, no matter how militarily strong they are right now.