Horus was never a good guy

I've seen people say that Horus' fall in the second book happened too quickly and that he needed more convincing to become evil and etc.

Horus was never not evil, we see this in the first book where everyone sees him as a nice guy, but he's always wearing a mask, Loken learns this when he joins the mournival, which is pretty much just a show of 4 different personalities giving him "advice" so he can choose the decision he wanted in the first place.

Had to massacre an entire planet? Well Abaddon convinced him but he was totally reluctant and definitely didn't want to do it.

That scene during Istvaan 3 where he gives his evil speech to the rememberancers before massacring them is an example of him taking the mask off, because he doesn't need it anymore. That's the real him, everything before was just show.

As for betraying the Emperor, he was already ambitious and disagreed with how the Imperium was being run, a general trying to usurp the state is a pretty common occurrence in our history and wouldn't even need Chaos to happen.