Well, he was said to use knowledge for good and he tried to and almost succeeded in killing Morgana which isn't good IMHO. So he simply wasn't wise enough to use this knowledge of the future correctly as a warning and in doing so made it happen. If it was Tale... it might have happened very differently. That's my take on it anyway. Gaius said that Crystal cave was the source of power for Tale... in the past and that implies that if you are wise enough you can harness this knowledge.
Morgana another question, her visions might be ones displaying events with highest probability and it might be that majority of actions taken will lead to the same result, but as in any probabilistic world it doesn't mean that everything is set in stone anything could happen just with different probabilities. And she is also young and have no experience in dealing with such premonitions.
And even when discussing show - I'm not interested in watching over adventures of mindless and powerless pawns of destiny it's more fun for me to assume that heroes have freedom of choice and that's why I would always go with an explanation that leaves enough room for it to exist.