Can someone please explain to me like I'm five how Britain and France weren't the good guys in WW1?

Europe, especially the imperial powers in the 19th and early 20th centuries, were and are a community of vampires, ghouls, jackals, demons, serial killers, and thieves.

The most telling part of the conflict was that despite Austria-Hungary initiating the conflict, Germany took most of the "blame" and was maid to payout reparations and had its colonies and industrial sectors and provinces swallowed up by the other European powers.

The causes stated by Lenin, in Imperialism: Highest Stage of Capitalism were:

Imperial expansion & imperial competition - a need to acquire new markets, materials, colonies, and growth. These empires were driven by monopolies and capital accumulation/centralization created existential rivalries among the European powers. Especially between Germany and Great Britain. These two powers considered themselves existential enemies since Germany had quickly industrialized and become on par with British industry in a short time, creating fierce competition in the market. These competitions justified and necessitated the need for new territories, raw materials, etc.

I can't remember much about Lenin's understanding of how finance capital's contribution to WW1, just that it was fuel to the fire of the overwhelming tinder that the need for new markets and resources made.

WWI as viewed by Lenin was an inevitability of crisis that capitalism produces. There needed to be periodic wars or catastrophic events that opened up new markets and populations open for domination, labor and resource extraction, and dominance of capital. I think the last great event like this was the dissolution of the USSR and the shock and awe policies that plagued Russia in the 90s that saw its industrial sectors siphoned and sold off.

WW1 is shocking in its cataclysmic damage and depraved indifference to the huge loss of life, but such a conflict was all but inevitable given that these were conflicts that capital needed to accomplish to break through to acquire new wealth and profits. The modes of imperialism that Europe had practiced were on this trajectory.

Ultimately the war weakened European powers, set itself up for WW2, and let the US readapt and manage the way these powers extract wealth and capital from its former colonial subjects and other states in the developing world.

Something by Rosa Luxemburg:

The Imperialism of all countries knows no “understanding,” it knows only one right – capital’s profits: it knows only one language – the sword: it knows only one method – violence. And if it is now talking in all countries, in yours as well ours, about the “League of Nations,” “disarmament,” “rights of small nations,” “self-determination of the peoples,” it is merely using the customary lying phrases of the rulers for the purpose of lulling to sleep the watchfulness of the proletariat.