He had his right to leave.
But more importantly English FA would have preferred for him to do this anyway. Having signed a reputed coach for a lot of money, they were left with a hard decision to make as he didn't do anything for England. Yes, England managed to qualify to the knock out stages in all the competitions under him....but they have lost direction. The future looks bleak. There have been coaches who have taken on the national coach job and made far more impact than Capello has, and i'm not speaking only in terms of result. You need to only look at Germany and Spain for examples. It's a good thing Capello took that decision out of their hands, because they weren't going to make it.
Capello treated this job as a paid holiday. England were a joke for the better part of the time he has managed and he has not addressed one issue well. He hasn't managed to replace Terry and Ferdinand well. He hasn't managed to replace Gerrard and Lampard well. He hasn't found a striking partner to Rooney yet. England still don't have any threat on the wings. The later two were problems when he took over, and it hasn't been addressed. The former two were obviously going to be problems as those players were 30+ when he took over, he didn't address them either. I don't know what he has done....
England would have been better off (financially at least) if they had employed Mclaren for the job.
Whoever takes over the job will have to do things from the scratch.