Apparently, there was a great many variation in choosing or getting a nom de guerre or 'war name', as Arthur Kollie for example went by the name "Human Garbage", "who insists on going by his war name, Major General Human Garbage", after the war.
(Cf — Danny Hoffman: "Monrovia Modern. Urban Form and Political Imagination in Liberia", Duke University Press: Durham, London 2017. doi, p33. and doi)
Others were called "Bush Cow" or "Iron Jacket", "General Mosquito Spray", "Chuck Norris" "Devil", "White Flower", "Rambo", "Red Banana", "Fuck My Dog", "Cow Poo Poo", "Top Bride", "No Bible", "Black Puzz", "Gen. Don't Blame Me" and so on.
(Cf — Republic of Liberia: "Truth and Reconciliation Commission", Volume 3: Appendices, Title XII: "Towards National Reconciliation and Dialogues: The Palava Hut or Peace Forums.", 2009. (pdf)
Blahyi had a reputation for being more brutal than other military leaders. Everyone knows his nom de guerre, which he says he will never lose: General Butt Naked. He was a cannibal who preferred to sacrifice babies, because he believed that their death promised the greatest amount of protection. He went into battle naked, wearing only sneakers and carrying a machete, because he believed that it made him invulnerable — and he was in fact never hit by a bullet.
— Jonathan Stock: "The Penitent Warlord: Atoning for 20,000 War Crimes. Joshua Milton Blahyi now visits the families of his victims to seek forgiveness", Spiegel/abc news, 2 November 2013.
But according to the autobiography of K Riva Levinson, 'Mr Peanut Butter' seems rather simplistic to explain for his name choosing:
Speaker of the House, Edwin Snowe, accused of stealing millions of dollars from a Liberian oil company, Senator Adolphus Dolo, known during the war as “General Peanut Butter” after his favorite food and accused of eating his victim’s body parts […]
— K. Riva Levinson: "Choosing the Hero: My Improbable Journey and the Rise of Africa's First Woman President", Kiwai Media, 2016, p151. (quoted after: — Dr. Fred P.M. van der Kraaij: "‘Choosing The Hero – My improbable journey and the rise of Africa’s first woman president’ by K. Riva Levinson", Liberian Perspectives, 10 October 2016.)
This seems to be even his own explanation for it: that he choose this as his wireless call-sign — or was assigned this name when responding to his then superior in the NPFL to a question about his favourite food when the time came to get a call-sign for use on radio, according to his own statement on video:
He said by what call do we — you know — come on with? I said: "Well, you tell me!" He said what's you favourite food? — I said 'peanut butter'! That's how, that's all, how this whole thing 'peanut butter' came about.
— Public Hearing Videos : "Sen. Saye Taryor Adolphus, Part 2", 2008-09-10, College of Liberal Arts - Ivan Allen College (IAC), The Sam Nunn School of International Affairs, Truth and Reconciliation Commission of Liberia, Public Hearing Videos, Georgia Tech Library.
Starting with his story about "selling 'Snickers'", to the point at timecode 17:41.
With this Peanut Butter coming in ever useful again:
To capture his generosity, Dolo used the slogan, “Let me butter your bread” during the election campaign. This phrase was particularly clever considering his nom de guerre had been General Peanut Butter.
— Anders Themnér & Roxanna Sjöstedt: "Buying Them Off or Scaring Them Straight: Explaining Warlord Democrats’ Electoral Rhetoric", Security Studies, 29:1, p1–33, 2020 DOI: 10.1080/09636412.2020.1693617
As down-to-earth as something like peanut butter sounds, a perhaps deeper psychological and maybe even legalistic perspective might be:
In fact, rebel commanders do seem to have an obsessive need to self-define — see for example General Butt Naked (née Joshua Blahyi), General Peanut Butter (née Adolphus Dolo), and Bosco ‘Terminator’ Ntaganda – and are not likely to welcome an internationally-assigned identity.
— Kate Cronin-Furman & Amanda Taub: "Lions and Tigers and Deterrence, Oh My: Evaluating Expectations of International Criminal Justice", in: Yvonne McDermott & William A. Schabas (eds): "The Ashgate Research Companion to International Criminal Law", Routledge: New York, London, 2013. (doi, p447.)