It has been 11 years. I don't think you have much of a chance. Also, he was threatened. There is no word of any action being taken against him. Did the one(s) threatening him hurt him? Is his death 2 months ago related to the event that took place 11 years ago? If his death is not related to what happened, there's nothing you can do. People die every day. I'm REALLY sorry you lost your dad because that's horrible, but why bring up past events now? If they didn't do anything to him (other than verbally threaten him), then they have nothing to do with him dying 11 years later.
You should have asked for the gas station's assistance as soon as it happened (or your dad should have) and also filed a police report. The sooner you report things like that the better. No use in letting it just sit. If it's important, you need to report it. Threatening someone is 110% illegal, so yes, the individual(s) would have gotten in trouble for what they did. If the cops didn't do anything, you should have pushed it further.
Good luck with your grieving process. Losing a loved one is NEVER easy, trust me. Even though it's been 2 months, I'm sure it's still hard. Never be afraid to reach out and seek assistance from someone if you feel you need it.