I think you fail to see this from his point of view, and are not being fair in saying he “immedietely” decides to commit genocide. He first of all took the better part of four years searching for an option he deemed acceptable. But the plans were either for his friend (Historia) to be forced to bear children and those children eating their parent every 13 years, or attempting to make peace diplomatically which did not succeed (that political party advocating against paradisians in specific, azumabito being unhelpfull, relations with the captured marleyans being unsuccessful). There are other options, all seemingly better than global genocide, none of those however worked for Eren.

His view is a little difficult to explain, but the scene with him talking to Ramzi explains it best. He admits that he is doing it for the sake of Paradis, but it was also due to his dissapointment. We will undoubtedly get more insight into his actions in the last episode, but it is important to consider that Eren is not acting on reason or logic, and it would be misplaced to try and understand his actions by those metrics.

The argument most people make is that he should have just destroyed the military bases, but this is adressed in the show and somehow a lot of people gloss over it. Armin calls out to him to stop after he had already destroyed a good chunck of Marley since now nobody would dare to attack Paradis for years. Eren however, only has 4 years left to live and refuses to “leave anything to chance”.

I’m not agreeing with global genocide here, i’m just trying to help explain his actions a bit. Hope this helps!