Jean would be not guilty for reasons already mentioned but Cyke is a tougher call.
1. The Avengers were operating with governmental authority so the claim of self defense fails when it comes to Cyke. It's not self defense when the authorities come to arrest you and you resist and in so doing cause someone's death. Doesn't matter how just or good you think Cyke was behaving up to that point.
2. Cyke courted the power of the Phoenix. This is clear because his plan involved training Hope to assume that power. He courted that power knowing the dangers involved as he was intimately aware of how that power had corrupted Jean.
3. When the power passed to him and the P5, he made no effort to remove such power from him. Instead he moved forward with plans to remake the world as he saw fit and it was clear that he was not out of control. Again being intimately aware of the risks of such a power he ultimately decided to use that power for his own ends and in doing so assumes responsibilties for the outcome of that decision imo.
5. The conflict between Cyke and the Avengers was foreseeable by him. Cyke knew the avengers and other Xmen feared the power and he knew they would oppose him. Further he knew the Avengers were acting at the behest of the US Government. He made the decision that he would ignore all of that and pursue his goals using the PF and he cant now claim, he didn't know this would happen when he faced opposition right up until the power was thrust upon him.
So based on the above he would be guilty of voluntary manslaughter at a minimum and possible 2nd degree murder. Regardless of how he came into the power, it is clear he consciously choose to use it for his own ends knowing full well agents of the government would oppose him and kmowing full well the corrupting influence of said power.