Slope is very bad to use, partly because the slope of a vertical line is infinite. Use vectors:
vel_x = bullet_speed*cos(angle)
vel_y = bullet_speed*sin(angle)
(vel means velocity)
Then every frame just update position by the velocity (or if you're using delta times update position by velocity*delta):
pos_x += vel_x
pos_y += vel_y
This way the bullet travels at the same speed in any direction.
btw the vector [cos(angle),sin(angle)] is a unit vector (length of 1) in the direction of the angle, so it moves the bullet by 1 unit in that direction. Multiplying by bullet_speed makes the bullet move bullet_speed units in that direction, which is what you want. It's probably worth learning a bit of linear algebra for this stuff.