When past evil Kamma gives its present evil results, the perpetrators (Devadatta or the hired thugs who killed Moggallāna at the behest of the heretics) are not doing it because of the past evil kamma of the victim, otherwise they would be free from any consequences of their own actions. They are driven by their own evil kamma to do evil deeds. They, in turn, were captured and executed by the king, as were the heretics who hired them. All experienced the vipāka of their evil deeds and were reborn in hell after death.
Thugs who are willing to kill anyone for a fee have a long habit of doing such evil deeds. Their will is neither completely free nor completely compelled, but conditioned by previous causes.
All of the royal archers who were sent by Ajātassattu to kill the Buddha due to the urging of Devadatta, did not carry out their initial intent. They were converted by the Buddha's great compassion and skilful means, and all became monks, so that plot came to naught. Likewise, the drunken elephant Nāḷagiri was tamed by the Buddha's mettā.
The Buddha experienced several negative results due to past kamma bearing fruit. Due to insulting the Buddha Kassapa as a "Bald headed recluse," he had to endure six years of extreme asceticism, whereas not all previous Buddhas had to do the same.
The law of kamma is not fatalism. There are light evil kammas (ahosi) that expire and never give results. There are medium kammas that may be diverted by wholesome deeds done later, there are heavy kammas the results of which can be mitigated, and there are heavy kammas that must give their results.
The idea of free-will implies the presence of a self or soul who is free to decide whether to act or not. In fact, our decisions are conditioned by many factors. Psychological research has shown that merely asking someone to hold a warm drink in their hand instead of a cold drink, can make them feel more positive about a person they talk too, influencing their decision whether they would give that person a job or not.
• • • • (Upasampadā: 24th June, 1979)