Why is my sink's drain pipe connection angled?
This looks like it's a renovation, or maybe just an island sink, because of the anti-siphon valve (aka air admittance valve (AAV), aka cheater vent). Either way, it's fairly sloppy plumbing by the looks of it. 

I'll leave aside the use of the AAV as I've never seen one like that: in many jurisdictions they're not allowed at all, and I've only ever seen ones that must come above the trap by several inches, or even be above the top of the sink. 

The trap you linked (SP10V) is actually designed to swivel, presumably to connect to a pipe coming in on an angle:



You should be able to rotate around that union and make it straight, then connect the rest of the trap from there. 

If the trap doesn't line up with the sink, however, the best course of action would be to make the riser pipe straight. Cut it off near the bottom, then using two 45° elbows you can shift it over slightly and allow it to line up. When you're doing this, it will probably require a few tries so connect the trap and dry-fit everything, and make sure it's nice and straight before you glue. Forgive my crude mock-up, but something like this: