"Clean Karma" by Wendelin Jacober is licensed under CC BY 2.0
Destiny and karma are enormously complex concepts. Nevertheless, I think there is a clear difference between them.
I would define karma as the price to pay for one's life actions. The human being is concious of its own freedom; its liberty to think, decide and act. We, as human beings, can decide to accomplish no matter what enterprise. Thus, only at the end of each one of those actions, would we be sent our own personalized invoice. Nevertheless, this bill is not necessarily going to be a must-pay receipt. Moreover, it could be some sort of gift instead. It depends on what we decided to use our freedom for.
I insisted on using the word "freedom" for a reason, and that is because freedom will be the argument through which I am going to explain why destiny could be just the reflect of people's decisions. Not until we carried out our actions, will we receive their consequences in our lives. Hence, it does not depend on what is "written", but on what we freely decide to do. Different religions talk about the free will of the human being. This is another argument supporting the thesis of karma instead of destiny. If we think about it, what we decided to do in some situation could be written already somewhere. Because it was written that we would decide that. Nevertheless, this rule takes us nowhere. There must exist some place for creation. It is impossible that we don't ctually decide our lives if we pay the consequences of our decisions.
Even so, we can also think that no matter what we end up doing or the reasoning behind any of our actions. It could be written already that we would think and decide that way. Not precisely supportive I am of this thinking, as believe I in the free will that leads the non-written destiny of universe. Nonetheless, I admit there are reasons to believe in destiny as well. Thereby, how can people have (dreaming or awakened) experiences where they see the future? The most standard example of this: someone dreams that a person is going to die soon. Whereas, there are moreover very positive experiences of this type. For instance, someone who views a friend in a future where he is surprised himself of the spiritual he has become in. In any case, this argument breaks the karma theory.
Hence, the question is: is it actually necessary to choose between karma and destiny? What if they both exist? This might be possible. A reason to believe it is that consequences of karma still take place while people continue to see other's future. As simple as this. Nevertheless, not this whole reasoning would be complete if didn't I mention how could this be possible. Even so, I need to clarify that I am not a scientist. And I don't believe what I am going to say as true. I'm just someone trying to figure out how universe works while taking his daily coffee.
Firstly, we should seriously take the possibility of several realities existing at the same time. This would explain destiny and karma at the same time. We face consequences of our acts depending on the reality where we are. In the reality number one we could take the decision one and in the reality number two we could decide to take the decision two. Different reality, different decision, different consequences, and even so, all of them are taken and all of them have their consequences at the same time in the different realities simultaneously existing right now. This could also explain why people see other people's future. Their future in this reality.
I hope this was clear enough in its complexity. However, I think that it will take the human being some hundreds of year more to actually understand or be able to answer those questions.