Unlike WW2, where the Nazis were clearly the "big bad guys" (imperialism,fascism, and all that came with that) , I believe that Germany's representation in WW1 of being "bad guys" was probably due to entente winning and propaganda that comes with that and also the 2nd world war that definitely tainted Germany's reputation heavily later on in history.
I could be wrong, but the only reason Britain joined the war was their alliance with Belgium and the Schlieffen plan, giving Britain the opportunity to stomp the mainland European world power, and crown the UK as the only European power. This could say that they were the "bad guys" as they joined to advance their imperialism and remain the world super-power.
Sorry if this is a FAQ. I always wonder why people shit on Germany in WW1 and how this is probably due to us British and colonies/USA/France (I'm from New Zealand) being the victors, and the whole "the victors write history" thing.
"giving Britain the opportunity to stomp the mainland European world power, and crown the UK as the only European power"
This isn't an accurate understanding of why Britain joined the war. The UK was concerned that there not be a single European continental power (in this case, Germany). Longstanding British policy was to maintain balance of power between the European Great Powers, and Germany's aggression (and it should be emphasized, it was acting aggressively) put this in danger.
Also Britain was far from "stomping" on anyone in 1914. The British Expeditionary Force, while it would grow to a massive army in 1916, as in 1914 a force of a couple hundred thousand in six divisions, and was all volunteer. It played a crucial role in the fighting that year, but we need to remember it was nowhere near the size of the French or German armies at the time, and was something like a tenth the size of the British army on the Western Front in the later stages of the war.
As for the "bad guys" part of the question: I really hate this term, especially when it gets used in military contexts (and it does get used - you can find plenty of contemporary examples relating to Iraq or Afghanistan). It's a very bad way to understand military conflicts.
World War II is perhaps an exception, largely because it was an ideological conflict and because Western states like the UK and US considered Nazism (rightly, in my opinion) to be a threat to civilization even before the war started and widespread Nazi atrocities were committed. But even in World War II, we need to be careful with "good guys" vs. "bad guys" rhetoric. Was Stalin "good" because he was fighting the Nazis (and remember 80% of all Germans killed in the war were killed on the Eastern Front)? Were the Americans "good" even though an eighth of Americans told Gallup pollsters during the war that they supported killing every Japanese man, woman and child? I should emphasize that I'm not trying to claim that everyone is bad or it's all relative, just that if we are trying to evaluate the morality of historic actors, we need to examine the morality of the actions of individuals and institutions, or even the morality of causes, rather than "sides".
Anyway, back to World War I. Clearly wartime propaganda on both sides painted the other in extremes. Both considered the other to be a threat to civilization, although in general British and subsequently American propaganda was more effective in winning public support.
However, after the war there was something of a revulsion against propaganda on both sides. There was a widespread idea in the US and Western Europe that the war had been a pointless, tragic waste. The Nye Committee in the US Senate infamously pinned the blame for the war on "merchants of death", ie arms manufacturers and banks for promoting war for profit (this isn't an explanation taken seriously by mainstream historians, by the way).
After World War II, especially in the 1960s, this idea of the First World War being a senseless waste was revised again. There has been a gradual move towards seeing Germany as the aggressor in the conflict, as I mentioned above. The German government, egged on by the German military, essentially saw the July crisis as a means to strike pre-emptive attacks on its opposing alliance: thus their decision to mobilize the German military in response to Russian mobilization (which Russia was doing in response to Austrian mobilization, which itself was doing because of German guarantees of support), and the decision to attack France (and along the way neutral Belgium) before it could properly fight Russia, is not the sole factor in causing the First World War, but arguably was a prime mover in turning a Balkan Crisis into a World War. Which again, is not to say that the Central Powers were "bad", but that they and especially Germany do hold a large share of responsibility for acting aggressively and causing the spread of the war.
You might get an answer to this but someone who knows what they are talking about is more likely to respond if you expand on the 'bad guy' question. History is a soft science but it still follows a process. Arbitrarily moralizing about good and bad might make good narratives for nationalists but it isn't professional history.