What's relative motion got to do with it?

1. Yes, it does. Relative to the outside observer, the bullet before it is fired is actually moving at 100mph, together with the car. Firing the bullet and accelerating it to 100 mph relative to the reference frame of the shooter cancels out the 100mph velocity that the bullet has relative to the outside observer. Thus, to the observer, the bullet will drop straight to the ground.
This was experimentally verified by Mythbusters. (The only thing the Mythbusters can not break are the laws of physics, thank Zeus.)
2. I doubt it. The police officer can be considered part of the shooters reference frame, since s/he is moving at the same velocity. Furthermore, it would have been your intention to hit the police officer with the bullet. That you exploited a certain aspect of the laws of physics for that is irrelevant.

In terms of the bullet’s speed, the speed of the cars is irrelevant, since they were going the same speed. The bullet still hits the officer at about 100 mph. As a matter of fact, the bullet would hit at a higher rate of speed than normal due to decreased wind resistance.

@ragingloli Is correct. The observer will see the bullet fall to the ground. If the trailing car hits the bullet before it hits the ground he will hit it at 100 mph

2. “You pop out of the sunroof and point your weapon backwards to take out the cop.” If he gets shot and dies, you’re charged with manslaughter at least.

@ragingloli has it correct.

@ragingloli Has it. Just to add a bit:

If the stationary observer didn’t see the lead car and only saw the bullet and cop, than it would look like the cop drove into a falling bullet.

@roundsquare Exactly. That’s what I was trying to say above, but you said it better.