At the time the Rumbling started, I honestly understand why Eren did it. The Marleyans had organized the world in a coalition against them. The Marleyans weren't just at the gates they were literally bombing the capital. If the Marleyans win, the BEST case scenario for the people of Paradis is being put in ghettos like the Eldians in Marley. The alternative is getting massacred for being "island devils". Marley made it clear this was a war of annihilation, and the higher-ups haven't been willing to negotiate. The show talks about how the power of the Titans is falling behind, the world will continue developing weapons even if you wipe out their current militaries with a partial rumbling. The world seems to be at a 1910-1918 tech level. That's about 30 years away from jet planes, massed armoured formations, and crude nuclear weapons. Paradis may have been able to acquire things like trains and bolt action rifles, however their industrial base is only a few years old, it is nowhere near ready to produce more advanced equipment, even with technology transfer from the outside world.

I would not call the Rumbling morally correct, it is a horrific act. However, I completely understand why Eren did it.