Not every "to" marks an infinitive:
- on their way to the store
- on their way to victory or defeat
- on their way to doing something
This is the ordinary preposition "to". In general, it takes an object that serves as a destination or target. This may be more obvious when the object is a simple noun like "store" or "victory".
When the object is a gerund or a gerund phrase, we still have the same ordinary kind of prepositional phrase. We can treat this "to" the same as we do when the preposition is followed by a simple noun.
Those countries are killing, or are going to kill, innocent civilians.
This is a reasonable paraphrasing of the tweet. This is probably the structure that you expected to see -- a structure that uses the infinitive-marking "to".
As a native reader, I find the grammar of the original tweet to be perfectly ordinary. My paraphrasing is also perfectly ordinary. There's more than one way to be grammatically right.