Rubbish. First of all, the Federation doesn't have the death penalty anymore (unless Fajo has gone to the Talos Star Group, which I doubt). Secondly, even if he did deserve the death penalty, would Data do it himself or would he make sure that Fajo was tried by a jury? Data never seemed to be the type to dispense summary justice. Thirdly, Data had the means to contain Fajo! I've explained this many times.