What were key factors in Japan becoming a fascist state leading towards WW2?

I actually wouldn't say Japan ever becomes a fully fascist state in the same way European fascist states do, but I'll explain that as I go.

Right, I'm going to start a seemingly long way back with this one: 1853. That's the year the American Commodore Matthew Perry arrives and quite literally forces the Japanese at gunpoint to open up their ports to American trade. The samurai, the military-feudal elites of Japan, which for centuries had been a largely inward-looking society with little need to interact with the rest of the world, are well and truly terrified by this. In 1868 the "Meiji Restoration" takes place - modernisers come to power determined to turn Japan into an industrialised, centralised nation-state like those in the West. And to a large extent, that's what happens. Japan's economy becomes an industrial capitalist one at an astonishing pace, military and political power becomes heavily centralised and bureaucratised, and the country becomes a constitutional monarchy with a parliament (though not with universal suffrage yet).

This is in large part done due to a considerable fear of being conquered or subjugated by a European power or the USA that begins with the events of 1853 and continue as they see the way the European powers carve up parts of Asia between them. This creates a determination on the part of many Japanese leaders to become a great power on a par with the European powers and the US, and to have its own empire as part of that. Many also see an empire as potential defensive strategy to safeguard Japanese independence. One of the most prominent articulations of this idea is that of Yamagata Aritomo (who was both a Field Marshal in the Army and twice Prime Minister), who said that Japan should have a "line of sovereignty" (i.e. 'Japan proper') and a "line of advantage" (i.e. a buffer zone to defend 'Japan proper'). This danger of this reasoning is that people start insisting on the need for new buffer zones (like Manchuria) to protect older buffer zones (like Korea).

However, expansionist ideology is by no means the same for all Japanese imperialists. Leading civilian politicians and senior military figures are largely conservative and pragmatic in their approach, in many ways wanting an approach similar to European colonialism; Japan's empire should primarily be for economic benefit; expansion should be pursued cautiously and gradually, and de facto, indirect control of other countries should often be considered preferable to direct control.

However, there also emerges a movement of junior army officers, often spurred on by populist nationalist media and fascist-esque political ideologues like Kita Ikki, who see expansion as being primarily about Japanese greatness (and, somewhat bizarrely, about 'liberating' Asia from European control) and believe that it should be pursued aggressively as a matter of nationalist ideological principle - even sometimes in cases where conquered territories became economic liabilities. They also generally don't like the conservative, largely bourgeois Japanese establishment and want something more like a mass, racially-defined, military-fascist regime for Japan, only with the Emperor as the supreme leader rather than a Fuhrer or Il Duce (this is actually quite ironic because the Emperor really didn't like the fascist types much).

The fascist-esque junior officers never really end up taking power - they try a couple of coups but they fail, so I wouldn't say Japan ever became a fascist state in the same way Germany and Italy did. However, it was heavily influenced by fascist elements within the military - the conquest of Manchuria was largely undertaken by the Kwantung Army (the Japanese garrison initially created to defend Japanese dependencies in eastern China) on its own initiative, rather than on orders from Tokyo (indeed, the government in Tokyo actually tried unsuccessfully to hold them back). The actions of the radcialised young officers also enable the senior military leaders (who can more effectively restrain and control the former) to gradually eclipse civilian politicians in terms of political power in Tokyo, which unsurprisingly is accompanied by a greater militarisation of Japanese daily life.

That's certainly not everything to be said, and I'm sure others can add more (and more specialised information), but I hope it'll do as a start. In terms of reading material, I'd recommend W. G. Beasley, Japanese Imperialism 1894-1945, though there are also a few general histories of modern ("modern" here meaning "since the Meiji Restoration") Japan that give a pretty decent overview that I can recommend if you want?