Was Cromwell solely responsible for the regicide?

Did Cromwell rig Charles I's trial and force the regicide through Parliament by himself?

No. Cromwell had major influence, but Parliament, Fairfax's Army, judges, councillors, and other officers also acted independently.

The account disputes the idea that Cromwell alone bribed, intimidated, and commanded every institution involved in Charles I's trial. The New Model Army was formally commanded by Thomas Fairfax, while Parliament, London's trained bands, judges, councillors, and other senior officers retained their own authority and choices. Some judges withdrew rather than participate, leaving a court more inclined to convict, and survivors later had strong incentives to describe themselves as coerced after Charles II returned. The Council of State itself had forty-one members chosen by Parliament, needed nine for a quorum, and was initially chaired by John Bradshaw, who had presided at the trial and signed the death warrant. Cromwell supplied military power and became increasingly dominant, but he operated within a broad revolutionary coalition rather than as its sole author. The wider point is that Cromwell accumulated authority within this dispersed system over time, partly because others transferred burdens to him, rather than controlling every revolutionary institution from the outset. The Commonwealth Navy's commanders and reforms emerged from the same distributed political-military system.