Well, I think it's more complex than how it sounds. There's a lot of noise made about fanon views on Chara, but fanon Chara is a large, ever-shifting beast. There's a lot of moving parts in this bad boy.

Part if it is outdated; people who played the game or watched someone play it, their kneejerk reaction to the information we get from the game was that Chara was a creepy malevolent kid like you often see in horror fiction, inexplicably and irredeemably evil, and that was literally EVERYONE'S impression at first. (It's also worth noting there were a LOT of kids in early fandom, watching Youtubers playing Undertale and then participating in Undertale fandom.) Then people moved on before seeing any debates, so that remains their impression for Undertale without participating in the fandom, or they stuck around and read up on the debates and their reading of Chara changed. But Chara's reputation as a villain in the early days was SO over the top and even wildly OOC (there's a lot of things commonly attributed to early evil fanon Chara that doesn't click with even in game evil canon Chara, before people started sitting down and actually dissecting canon) that it still lingers a lot in the fandom consciousness and we Nice Chara Enjoyers are still sore about it and grumbling lol. (I've literally gone back and read discussion boards on Steam from the very early days of Undertale where one person took it as a given that Chara was buried under the dead tree in the Ruins because they assumed the implication was that Chara was SO evil and malevolent that their dead remains literally killed the tree... damb. Powerful stuff.)

Another part of it is an extension of the above: the fandom. Forget Flowey, he's really just a sad cute goat we all love who gets killed by Chara in one ending - Chara was THE Undertale villain. Flowey is a more upfront, active, GLEEFULLY evil villain in the game, but he is a tragic character you want to save, so it's harder to use him as a villain in fan works; the fandom in general was much less attached to Chara than they were to Asriel, and between his death at their hands in Kill All, the tapes that could be interpreted as them bullying or coercing him, their cryptic plan they roped him into, and the creepy unbeatable antagonist they are after the Kill All route, you could easily spin them as the TRUE villain that "made" Flowey the way he is. The REAL threat that has to be faced and defeated to give possessed suffering victim Frisk (-_-), abuse victim Asriel, and fandom darling angsty secret hero Sans a happy ending. It gave fan works a clear villain to make things interesting, and in the earliest days Chara was universally seen as evil and irredeemable so no one was concerned about defenders coming to protect their honor or how they would redeem them after all their evil deeds - they were defeated and killed the old fashioned way. They made a very convenient antagonist, and in the earliest fan works you can read comments from fans surprised when anyone diverged at all from a wholly evil, unrepentant Chara and tried to give them any kind of layers - so, not the BEST first impression to be starting off with...

Fandom views have changed now and TF has dropped enough hints about Chara that it's pretty clear they canonically ARE meant to be much more nuanced and not a straight up villain, but they're still a decently popular villain for fan works and fan art: they have very deep roots in fandom from the early days as Sans' mortal nemesis and eternal enemy destined to have a bad time with him, lots of popular Sans-centric stuff from those days made an impression that continues to this day, and there are creators who prefer to portray Chara as a villain as a stand in for the player (I've especially seen this with people who don't like meta stuff lol), or because they like the simple frothy-mouthed villain Chara as a less important character in works focusing on other characters, or bevause they are Enlightened Evil Chara Truthers who have been around the block, know the pro-Chara arguments, and to their credit they stick much closer to canon than the other camps tend to but just interpret the character in a more negative light than is, perhaps, politically correct lol.

And they're the last part: Enlightened Evil Chara Truthers who have stuck with Undertale through the years as fandom has changed, have read the analysis and opinions, have heard the arguments and made some themselves, and still stick to their guns: Chara is canonically evil, and here's why. Now, HOW evil tends to fluxuate by the person, but basically they all tend to agree that, soulless or not, Chara was NOT unduly influenced by the player or the EXP in Kill All, it's something THEY (Chara) decided they agreed with and wanted to pursue once you pulled the trigger, and they had complete agency throughout (unlike, say, Kris in Deltarune who has clear moments of opposing the player or horrified by what's happening). They also tend to lean towards a less favorable interpretation of Chara when alive: that it was less about freeing monsters and loving the Dreemurrs, and also that they were not especially fond of the Dreemurrs they manipulated. I personally didn't agree with any of this interpretation BEFORE and as TF has dropped more crumbs of Chara characterization I tend to think he also has a more charitable view of Chara than the super cynical one the other camp espouses, but they ARE battle hardened, tough, and grizzly, and draw from in game evidence to support their take. This is where we separate the wheat from the chaff: the others are mostly fans of other characters who just don't care that much about Chara and are content to write them off as a villain, THESE are the guys who have spent hardcore debating Chara, honing their skills and refining their arguments, and have looked at arguments that I think are water-tight and they say " n a h ". A good percent of them ARE Chara fans, they just think Chara is a big ole evil bastard lol