(Please Help!)How can I get rid of this irrational fear/worrying that my parents will get killed?

Look, if everything you're saying is true, you've got some major issues. I realize you would like for there to be some quick-fix answer but it sounds like you've got some big things to deal with. The short answer is "quit worrying, they'll likely be fine". The long answer is you may need therapy, medication, a drastic change in diet, religion etc...there are plenty of people whose parents have dangerous occupations and/or are depressed but they are not obsessesed in the way that you are, so you need to figure out why YOU feel this way in order to stop. It's also possible that your fear could have nothing to do with fear (i.e. if you were ever raped, molested, beaten etc. excess fear could be your subconcious way of hiding from what the real issue is). Lastly, although I'm not religious I do believe in God, and I believe that when you pray hard amazing things can happen. Pray for happiness or contentness or a solution to your bad situation etc and REALLY BELIEVE that help can come your way and you may be amazed at the results you get. Whatever you end up doing, I wish you well and I hope you get to a better place.