Sauda Arabia uses executions even today to end the life of the terrorists and if this was USA or any other country the men would of been in jail and legal cases would be fought for a long time which would mean spending money and efforts to release the men and hope these guys were guilty, and not innocent men and it is sad these men were responsible for the loss of innocent people in terror attacks. I think that the punishment may be extreme but it can lead to others not doing this sort of crime and also not to oppose the government in this country otherwise, the punishment will be brutal and this action is being taken due to attacks by terrorist groups and also rebels who want to overtake the country. The country has been doing a good job in punishing the ones who are causing terrorist attacks and also hope the right people get punished which will be a warning to others thinking of doing this sort of crime as the country is very strict in enforcing the law.
Iran is just talk. They have no chance against KSA, so they don't prefer to engage in a direct war with KSA. Their best is stirring up sectarianism over the region to serve their political orientation/ideology. No doubt they've some armed militias and proxies here and there, but they'll collapse if we keep destroying them.