Every cat has a different view when it comes to destiny. Some see it as evil; predetermined, while others see it simply as guidelines to help them along their paths. Destiny is perhaps both of these, in a way. There are some things we have no power to change. But not everything is so untouchable.
I could not avoid that monster accident. There was no way for me to foresee the consequences on my own. There is no way for any cat to do so, not even those in StarClan. You see, there is no real "future." Time will continue to pass, true, and a time will come that is later than today, but it is not the future. It is then the present.
I sense some confusion, allow me to further explain. Every situation must have an outcome. Some are likely to occur, while others are less so. Not even StarClan can see what will most definitely happen. We are gifted with the ability to see outcomes, and we are then able to identify the most likely outcome.
Most, if not all, cats have experienced StarClan in their lives. Whether it be through belief or even visual encounters, all cats are aware of us. We have quite the reputation, if I do say so myself. We allow ourselves to mingle with the Clans and help them as they struggle to survive. But we don't know their future. We only suggest what we believe they should do.
Take prophecies for example. "Lion and Tiger will meet in battle, and Blood will rule the forest." Firestar, somewhere in the back of his mind and the depths of his heart, knew that he had to fight BloodClan. We only gave him a little push. As for the last part... That was pure motivation. No cat wanted Scourge as a leader. Knowing that he was likely to win would only make LionClan more determined to make each blow count. It easily could have gone the other way, but StarClan decided to take that chance. It paid off, you'll notice.
Choice plays a major part in destiny. There are things we cannot do anything about. But StarClan is always willing to give a worthy cat a second chance. I was not destined, at first, to become a Medicine Cat. But as soon as I disobeyed Firestar, my fate changed. The tables can turn in the blink of an eye, in one pawstep, in every word spoken to every claw-mark on our enemies. Every kit born and every elder sent to StarClan. Every warrior slaughtered in battle and in those killed by disease. Every apprentice made and every apprentice given. Everything is a variable. One little thing could save the Clans or break the forest.

So be careful out there.

This, if you haven't noticed, is from Cinderpelt's point of view. I found her life sort of unfair, and I thought talking about how destiny can change would be good coming from her. Remember, this is just my opinion on "fate." I realize that, most likely, StarClan really can see the actual, certain-to-happen future, but this is how I've always thought of it.