Mateo Pérez Rueda was one internship away from completing a degree in political science. The 24-year-old also worked as a bicycle delivery rider and sold fruit salads and juice to finance his passion: the Colombian independent digital magazine El Confidente.

On 4 May he travelled to Briceño, in the western province of Antioquia, to report on the long-running conflict between the army, paramilitaries and dissidents of the Revolutionary Armed Forces of Colombia (Farc).

The next day, he stopped responding to his parents.

Three long days of agony followed, with relatives and friends pressing the authorities for information, until a humanitarian mission confirmed what many had feared: Rueda had been kidnapped, tortured and killed by one of the Farc dissident groups, known as the 36th Front.

His case became yet another symbol of the surging political violence that has reached its highest levels in a decade – and that has made the decades-long internal armed conflict central to this Sunday’s presidential election.

The vote will be a contest between left and right – and two entirely contradictory proposals for dealing with the war that claimed nearly half a million lives.

Colombia’s president, Gustavo Petro, who under the constitution cannot seek re-election, has backed the leftwing senator Iván Cepeda, 63, who is leading in the polls and is regarded as the architect of the government’s “total peace” effort to sign disarmament deals with all criminal groups. Many security experts consider the plan to have failed, noting that armed factions have taken advantage of temporary ceasefires to continue expanding, but Cepeda remains committed to the plan.

The two main challengers, the far-right lawyer and “outsider” Abelardo de la Espriella, 47, and the rightwing senator Paloma Valencia, 48, promise a return to all-out war as soon as they take office.

During the election period, there has been a surge in guerrilla attacks, homicides, kidnappings, forced displacement and massacres; and last year, the rightwing senator and presidential hopeful Miguel Uribe Turbay was shot during a campaign event and later died. The violence is widely seen as a reminder that although the landmark 2016 peace deal between the government and most of Farc dramatically reduced violence for years, it did not end it for good.

Subsequent administrations slow-walked the implementation of the settlement, while some Farc factions and other rebel groups refused to sign any agreement, instead growing in strength and size.

“Here in Antioquia, the war never ended,” said Jorge Rueda, Mateo’s cousin and godfather, who lived a few blocks away from the journalist in Yarumal, only 33 miles (53km) from where he was killed. Although the various rebel factions claim a political agenda, most of the violence is driven by competition over drug production, retail and smuggling (Colombia remains the world’s biggest producer of cocaine), illegal goldmining, logging and local corruption. “Here, the war is over micro-trafficking and another over the goldmines,” added Rueda.

On Monday, more than 50 people were killed in clashes between two Farc dissident groups on the opposite side of the country, in the southern department of Guaviare. Many of them were children and teenagers forcibly recruited by the crime factions.

Alejandro Chala, a researcher at the Fundación Paz y Reconciliación, argued that although the figures were high, the current moment was not comparable to the period before the peace agreement, when the homicide rate peaked at about 80 per 100,000 inhabitants; it now stands at about 26 per 100,000.

“The violence now is much more territorially concentrated, largely entrenched in the main areas where illegal economic routes operate … It clearly generates a lot of media noise, but it does not have the national reach it had in the past,” he said.

Even so, Espriella has argued that it is necessary to “save Colombia” from crime, while Valencia says that instead of “total peace”, the country needs “total security”.

Until recently Cepeda remained firmly at the top of the polls, with Valencia in second place; but in the past two weeks she has been overtaken by Espriella. With a large share of voters still undecided, the outcome is uncertain: if no candidate wins more than half of the vote, a runoff will be held on 21 June.

Valencia has been a senator since 2014, and is the granddaughter of the former president Guillermo León Valencia, and a loyal follower of the ex-president Álvaro Uribe Vélez, whose two terms between 2002 and 2010 were marked by an aggressive military confrontation with armed groups that produced limited results and became tainted by the “false positives” scandal, when innocent people were extrajudicially killed by the army and falsely labelled as enemy combatants.

Espriella is a criminal lawyer and millionaire businessman with a lavish lifestyle who owns wine and rum brands and investments in cattle ranching and real estate, but has never held public office. He is an admirer of Donald Trump, and has courted controversy during the campaign, telling a radio host that he is winning female voters because of the size of his genitals.

With his neatly trimmed beard, he has also modelled himself on El Salvador’s populist autocrat Nayib Bukele, who has imprisoned at least 2% of his country’s adult population as part of a controversial crackdown on gangs. Espriella promises to follow the Salvadoran mano dura (iron-fist) approach and build 10 maximum-security “mega-prisons”.

Sandra Borda Guzmán, an associate professor of political science at Universidad de los Andes, said: “He also carries some elements of this new counterculture against political correctness, in the style of [Argentina’s president] Javier Milei and Donald Trump.”

Despite Espriella’s openly declared admiration for Trump, the US president has so far refrained from endorsing either him or Valencia, unlike in other recent elections involving far-right candidates, such as in Hungary, Honduras and Argentina.

Guzmán believes one reason may lie in Espriella’s recent past: he spent years as the lawyer for figures such as the Colombian businessman Álex Saab, widely regarded as the main financial frontman for Nicolás Maduro’s regime in Venezuela, who was recently deported to the US by the acting president, Delcy Rodríguez.

“As a criminal lawyer, Espriella has long been linked to people prosecuted or extradited by the US and those close ties remain concerning for Washington. So he’s not really the ideal candidate for the White House,” said Guzmán.

She believes another reason why the US has not openly tried to interfere in the election is that “they eventually realised those interventions produce the opposite effect to what they want”, noting that Trump’s attacks on Petro – calling him a “sick man” and “drug-trafficking leader” – ended up boosting the Colombian president’s popularity.

Petro, a former member of a smaller rebel faction that signed a peace deal years before the Farc agreement, is Colombia’s first leftwing president. His approval ratings are widely seen as high for a president nearing the end of his term, something many analysts attribute to the expansion of the government’s social programmes and increases in the minimum wage, alongside falling poverty rates.

Despite losing his godson Mateo to the internal armed conflict, Jorge Rueda believes Colombia is doing better.

“I could say something different out of anger, but from the heart I believe Colombia has improved enormously in recent years … such as giving young people better opportunities so they don’t join the armed conflict. However, there are some regions that never improved,” he said.

In those areas, he added, the absence of the state allows criminal groups to take control and drive away any prospect of private investment. “That is why I think it is so important that Mateo’s case receives attention, and that his death serves to show that there is a part of Colombia still forgotten and that neglect is what keeps the war so intense.”