I'm trying to draw a vector diamond with a tutorial, but when I connect two anchor points the corner becomes extremely sharp. Why is that happening?
Here's an example of what's happening
That happened because you connected the two vertical lines, giving an extreme small angle, resulting in a very pointy tip. You could change the corner appearance in the stroke palette (e.g. round) to adress this. But you really only need to connect them if you want to draw a shape.