FARC took up arms after decades of legal struggle were continously crushed in the most brutal and barbaric fashion by the imperialist-backed state. Colombia had their own national-liberation, social-democrat candidate, Jorge Eliécer Gaitán, get assasinated in cold-blood right before the presidential election he was set to win, setting off 10 years of civil war known as La Violencia between 1948 and 1958. This armed conflict is pretty much where the precoursors to the FARC were born in the form of communist popular front groups that used a combination of legal and illegal (armed) struggle to fight for the peasants and workers.
After, some of these armed groups were able to push out state forces and form autonomous regions like the Marquetalia Republic. These were brutally crushed by the military, and the survivors, many of whom were PCC members (Communist Party of Colombia) formed the FARC. The point I'd argue is that there was no other place for the workers movement to go after decades of low-grade "drip-by-drip" violence, punctuated by massacres of peasants and workers. Armed struggle was the only way, which lots of people like to deny when they try to frame them as "narcoterrorists" or whatever.
Now for the history of the FARC during its existence, it's very messy since it is a case of guerilla warfare. I would say I think they provided a necessary counterforce to the neoliberal pillaging of the country. After the peace, this process has only intensified, and more social leaders are being assasinated by the state and pro-state paramilitaries, not less, including former FARC members who were part of the peace process.
edit: It should also be noted that FARC made attempts on the 80s to shift back to less militaristic tactics with the formation of the united-front style party Unión Patriótica, who also ran a candidate who came in third, Jaime Pardo, and who was also assasinated along with hundreds of other UP members, leading to another period of primarily armed struggle leading up to Project Colombia, which was more or less an extermination campaign by the US that the FARC wasn't able to withstand.