the 50 f1.8 is already a very fast and workable lens. as for purple fringing (seen on the two ears of the cat), it can be seen on images taken from lenses like 24L, 35L, 50L & 85L.... It is a characteristic of a fast lens at wide aperture opening. There are ways to overcome it.
You may have to understand the direction of light and the weakness of the lens to overcome it. But having it is not issue generally as it can be easily fixed with afew clicks.
Your picture had captured intrigued look of the cat, which is priceless.