Colombian here, I can give some context, at least from my perspective.
Current president is Petro, a left wing president. Cepeda is the candidate from there same party. Abelardo is the far right candidato that won.
Petro has done an awful job. I'll add some examples in a comment. Cepeda seems better than Petro, but lacks criticism against him, so people add Petro's shortcommings into Cepeda's candidature.
On the other hand Abelardo got sold as an outsider to the classical politics, as he came out of nowhere, and 1 year ago nobody knew who he was. He's actually been an ally of Uribe (our most controversial president in recent history; far right as well) and the traditional right, just simply not as involved in politics. He also got his fortune by defending and being an ally of criminals (think an IRL Saul Goodman). He's beyond awful and like OP said, a genuine scumbag (e.g., in an interview, he laughed while remembering how he used kill cats with explosives).
Some people were gonna vote against Petro no matter what. Some people really ate the lies that Abelardo, a man that got his fortune by allying with criminals, will fight really hard against criminals. Some people think both candidates were awful, and had to choose the least awful of both of them. And that's how we got him as our next president.