What if I take a laser pen and direct it towards the middle of two wide slits? Will I get interference pattern or just two lines? Do I need to scatter photons before they hit the slits to observe interference pattern?
The reason incoherent light is passed through a slit is to make it coherent . The light from a point source is forced to be coherent, the incoherent part is scattered off, or absorbed, at the opaque surface where the slits are formed.
Laser light is by construction coherent so no first slit is needed.
Scattering also often results in coherent waves and that is why we [may see interferences] with street lights; even see them in light patterns in sunlight .1 This video at about 4 minutes shows interference patterns from street lights.
Yes, you will see interference pattern, provided the slit widths and their separation are appropriate. Look here. If laser beam width is greater than the separation of the slits, the beam may split and might form interference pattern. It is to be noted that particles of appreciable size (coming in the path of laser) will also scatter light according to Tyndall effect.