Complicated to explain .
In fact, the sawtooth effect is due to your enemy that creates sprite going straight ( 90 ° angle in your case ) but with the number created and the rapid movement of your enemy , you have an effect of optical . If you put a speed lower sprite creation (using "every x sec : spawn bullet " , you can see that they will stupidly straight ) .
I think you should diminish the number of "bullet" and that you apply a path to your "bullet" to fix it and have a curve.
The start angle is defined by the direction of the enemy and as soon as the "bullet" is close to the edge of the area , it changes direction
I don't know if I explain myself properly ( English is not my native language )