Why is Morgan Le Fay portrayed as evil?

Morgan le Fay is not always portrayed as evil - she is a very borderline character. The tales in which she is evil generally just makes for a more interesting story: she plays the important role of an antagonist, and in some versions she takes Mordred's place so the story does not need to go far into the tale of Arthur to find a villain. Another reason is that people were often very superstitious of magic and sorcery, so oftentimes they turn her into a "bad guy". These are mostly the older versions. However... Morgan is almost one of the "good guys" in some rarer variations, but these are often not directly to do with Arthur. She is sometimes a healer, helping wounded travellers, or she gives questing peoples advice. Evidently, you have heard the eviller versions, but keep searching for other people's tales and you will discover some people favour her more as benevolent.