https://www.businessdailyafrica.com/bd/lifestyle/society/olenguruone-origin-of-famous-mau-mau-oath-2273544I doubt you great grandfather would just be so bitter to want to murder people that much.
The genesis of Mau Mau and many upheavals in late 40s and 1950s - was tractors and the italian POW.
Many kikuyus found themselves jobless, in penury and were send back to Lari/Kiambu - and that is where bitterness started.
My own great grandfather who I am named after - also fell victim to the tractors.
Mechanization of farming - saw lots of people rendered jobless - and it brought problems. My own great grandfather was kicked out of white highlands where he was crazing his cows - then when he tried to go to Narok - his cows were confisticated - and sold - he died in depression shortly.
British razed kikuyu workers huts in rift valley and told them there was no more tilling job - as tractors took over. From my grandfather - a few set up slums in Molo and such areas - while some tried to return back to central. They tried to get back their original land and the bitterness started.
Most kalenjin were giving out their bulls in exchange of pasture - they were similarly told to move out - as their bulls were no longer needed. Kalenjin moved to border of british farms and established rural slums - heavily populated areas on edges on Mzungu.
Kikuyus kicked from Molo went back to Lari - few tried to expand to Maasai land - British and Chief cracked down hard.
Now desperate, poor and vengeful they went for the chiefs.
My uncle who is named after my great grandfather hated kikuyu homeguards with a passion. he would confront them and his favorite thing to say was that they were too greedy and they will finish kikuyu goats due to their greed. As mau mau looked for local mau mau leaders to help them execute one the attack against Luka they settled on my uncle. he was a fierce leader and activist in the community. he gathered intelligence and provide mau mau with a list of all homeguards.
on the night of killings he led the mau mau soldiers and supervised most of the killings. by dawn hundreds of loyalists had been brutally murdered and their homesteads torched.
He was arrested shortly after the lari massarcd and charged with murder. before his cased was ruled he was offered a prebargain. he was asked to denounce mau mau and collaborate with the the British to save his life. he called his mum and told her that he had made the decision to die for the cause. he was hanged in Githunguri and burried in unmarked grave.