No doubt, sigma 35mm 1.4 art is canon killer.
Check some reviews, many people compared the two lenses. Though I'm not quite sure about the build quality that Sigma provides — at first sight yes, it's very good, but will it last as canon 35mm does?
I'm not sure I'd call it a Canon killer....it's a little bit sharper but that's about it. Also bare in mind that I've been using my 35L for the last 8 years (and it's paid for itself time and time with great professional images) where as this Sigma is new to the market. I'm happy with my 35L and see very little reason to swap to the Sigma. My Canon 35L has provided great images and will continue to do so.
If you rate a lens by optics alone then sure the Sigma looks great. But I would wager in a comparison that few could tell from an A3 print which lens was which. The Canon is far better built, has a far more reliable AF system and will hold it's value on the second hand market over the long term. Sigma AF issues are well documented, even their 120-300 has af issues.
You're wrong about build quality, the Sigma is much better, all of my 35 L's has been squeaky when squeezing the sides of the barrel, and two of them had the small plastic pins that holds the af/mf switch in place broken, leaving an open hole right into the lens. AF issues are a copy related issue, when you get a working one, AF is really good with the new Sigma's, in fact I JUST 20 minutes ago tried to activate all of the 61 points on the 1dx, instead of only cross type, as they didn't work well with any of my lenses, but the 50 Art they work great, so finally leaving them activated. As for the issue of the 120-300, they updated it a few days ago to work way better with the 1ds3 and 1d4.
Color and contrast as well as sharpness in corners are better with Sigma, but I completely agree with you that the IQ of the 35 L is really good, and the AF of that lens is simply fantastic. But color and contrast, build and ca correction needs a BIG update.