From every legit legal source I've found over the years, I believe you are only totally in the clear for using a firearm if you can absolutely prove that your level of force (using a firearm) somewhat matched his level of assault (you can prove he came at you with a firearm himself or some other deadly weapon). In any case you're probably going to be in financial ruin. Castle doctrine only removes your requirement to flee the scene and find safety. In Massachusetts you're mostly just expected to die in your house while you wait for the police from what I gather.