My son was killed in a home invasion, was this Allahs death date for him or was it interrupted by the killers. I need help understanding this.
Whatever happens in this world, happens by the permission of Allah. But doesn't mean Allah is making people do all bad thing that is happening in the world. Whatever way a man or anyone or anything can behave is determined by Allah. So a man can have number of good ways and number of bad ways allowed by Allah for him. So a man who is god conscious should from the good ways.
About your son's death, it happened because Allah allowed it happen. May be Allah will forgive your son's sins and admit him into paradise as he was wrongly killed. Once an event happen, it is Allah's death date for him. Also sometimes if Allah already decide someone die on particular place or date, that would definitely happen. So your son's death was one particular way that Allah allowed and it happened because the person who did it decided to go on the wrong path allowed by Allah. So he will have sin for doing that, may be Allah forgive him later on that is different issue.
First of all, please accept my condolences for your sad loss and grief. It must be a terrible time for you and your family.
As for your question, the answer has to be a "yes", (there is no other credible answer), i.e. it was definitely part of Allah's plan, and no one, not even home invasion murderers could change it or, as you put it, "interrupted"
If any human could change or interrupt Allah's plans, then Allah would not be Allah anymore, isn't it?
Millions of incidences, good or bad, happen every second somewhere on Earth, and if every "bad" incidence is an "interruption" of Allah's plans, then Allah's plans are being interrupted millions of time every second?
As for the question, what then is Allah's post-death "plan" is a matter for pure speculation, as no human is privy to Allah's plan, not even @Shafeek.
Of course we all hope that Allah will have mercy upon your son's soul and admit him into Paradise.