Are Motorcycles worth the risk?
No one can answer that question for you. The answer to that question is going to depend on the person asking it. The amount of risk assumed by one person isn't the same as the amount of risk assumed by another. How much risk one person is willing to take is different than another.
I have to say that you have taken the first step in realizing that there is risk involved in riding. To start with, think about your level of risk. As an 18 year old, you have a minimum of experience on the road (maybe 2 years of driving experience - in the big picture, very little) You might have a little experience riding a bicycle while hugging the right side of the road - but I'd have to say that it's more likely that bicycling will have given you some bad habits. Think about how you drive, and whether or not you've had any close calls (you have to be honest with yourself, or you're wasting your time). As an 18 year old, with no previous riding experience, you are probably at the extreme high end of the curve risk-wise. Your age bracket is over-represented in accident statistics. You might be exceptional, and have lower risks - most people who think they are, are not. What kind of car do you drive? Do you drive a little faster than you should sometimes? That will tell you a little about the risk as well.
As far as how much risk you're willing to assume, you have to look at your situation. At 18, its unlikely that you have children that depend on you, so that's a plus. Assuming you avoid other dangers, you probably have a long life ahead of you, which could be adversely affected if you seriously hurt, or ended if you kill yourself. Do you have life insurance? How about long-term disability insurance should you become injured? Also, good medical insurance will make a big difference, should something bad happen.
I am a Motorcycle Safety Instructor, and I make money from training people how to ride motorcycles. That being said, I think in almost all cases, it's bad for a teenager to ride a motorcycle. You've already had someone near to you end their life probably just making the dumb kind of mistakes that almost all teens make at one time or another. I know that all of us older people seem to be saying the same thing - it's because we remember what it was like being young. In most cases, the problem is that your brains don't understand mortality yet. Your parents have probably sheltered you from the worst of what life has to offer. It sounds like your friend's death may have given you a glimpse of that. I think it would be a nice tribute to your friend, and give his death a little meaning if you would learn from it.
But you are an adult, and you can make your own decisions now. Go take a motorcycle safety class (whether you're going to ride or not) - it will make you a safer driver, even if you don't ride. The best things in life are worth waiting for. Even if you don't start riding for several years, you'll still have a long life of riding ahead of you.