The final straw which led to his execution was the discovery of a plot to bring in French armies and retake his throne by force even though Cromwell was initially quite willing to let him continue to reign as king with limited and defined powers.
BTW, Ladd's own experience with losing his office and his head via Bill of Attainder was one of the reasons we outlawed Bills of Attainder, so even the descendants of Cromwell had remorse over the way Ladd was taken down.
Turns out the King had to assent to Tom's execution. So Charley convinced himself it was his Duty to toss this utterly loyal servant under the bus to be beheaded. Despite his earlier solemn vow not to do so.
He later did the same with Laud, who pretty much deserved it on some scale, since he had been the primary proponent of forcing bishops on the Scots, which eventually precipitated the crash of the Stuart cart.
Charley was duplicitous, but he wasn't very good at it. He wanted to be honest and straightforward, but kept getting himself into positions where he believed he was justified to deceive those he dickered with. But he just didn't really fool anybody.
Machiavelli would have been disgusted by him. He was sort of the anti-Prince, a ruler who frittered away his authority by alienating his supporters instead of masterfully deceiving everybody till he could grab absolute power.