Unfortunately a rather extreme example of the sort of CA you can get when using a superzoom and are dealing with very high contrast scenes. If I had bought the lens new and it showed that much CA I would be inclined to send it to the manufacturer under warranty, and ask them to do a propper check and alignment on the thing. This may actually be one of those situations where a repair, if possible, would be better than simple replacement. Since you would at least know the lens had been properly set up. Do you know anyone else with a Canon body that you can try the lens on, to see if it is a particular combination of lens/body, or just the lens?
If you bought the lens as a nearly new second hand item, then I guess you now know why they unloaded the lens on you. I would again suggest checking performance on another Canon body, just to confirm how bad it really is. If you bought second hand and have no real easy recourse to the seller, I would try talking to Tamron and see if they will help you out. If you ask nicely they might be willing to help you out, at least partially, with the cost of a repair. Many companies are willing to try to maintain good will.