Philosoraptor

Yes, but it was inevitable and humanity will cross even deeper lines

Heh, well, it really did pave the way to the Hydrogen bomb which was literally 1,000 times as powerful as the atomic bomb and could theoretically be built in indefinitely increasing power based on size.
Honestly though, Hiroshima and Nagasaki weren't anything too special compared to the other cities that the US bombed with conventional weapons (like Tokyo) from a purely statistical standpoint (other than that it only took one bomb of course).

Tbh either way the same amount of people were gonna be killed.
Either Japan is gonna purge 3 countries from existence or they get bombed

I'd say both... They did cross a line, and it also paved the way for more dangerous weapons. I guess it's human nature to keep stepping over borderlines. The question is, what the final borderline will be before we doom everything in existence? Or did we already cross that line is it just a matter of time before we notice?

I don't think there was a line that have been crossed as it was born out of necessity

The Japanese were not only fanatical fighters, but in general would not surrender. Moreover,, the Kamikaze attacks shocked the Americans even more because of their willingness to crash into their ships. It was a bloody fight for the US all the way to Okinawa. If they were to invade the Japanese mainland, they can expect much more of that.

It is because of that, the government sought for weapons that can end the war quickly

If there is a line and it hasn't been crossed, I think that it will be crossed if we again use the atomic bomb and the bombs that were created after it in battle.