How to tell my parents i want a dangerous job?

I don't think this is really that dangerous of a job when compared to being a commercial fisherman or coal miner. I think most parents would like to hear their kid wants to do something like this.

Tell them that if they don't let you that they'll be the first case you work on. I'm sure they don't want to draw their last breath anytime soon. Or, if you don't want to threaten them, I suppose there might be other methods. I'm sure detectives get a lot of training, and know what they're doing, so it's not like you'd be going into the job unprepared or completely defenseless. Ultimately, it's your decision and you should be the one choosing what you want to do with your life. Personally, I think my first suggestion is the most effective, but you might not want to become one of those corrupt cops before you even start! But, if it's something you'd consider, I know a guy...

How is that dangerous? Being a frontline cop is dangerous but you're dealing with the aftermath not the spark.

Just tell them straight out and let them know that you are completely serious and prove to them that you are passionate about this choice.