The basis for this technology already exists, it is called an Electrolaser and it works as follows:
You fire a laser into the air, and take advantage of the fact that lasers diffuse in air by stripping electrons free of any atoms that they come into contact with. This creates a directed channel of free electrons, also known as Plasma, which is one of the best electrical conductors in existence. You then create a surge of electricity at one end of the plasma channel, which conducts down said channel like it was a live wire.
Applied Energistics is the primary US contractor working on this technology, and they are currently working on designs for everything from portal denial (a stationary system that can remotely shock people trying to pass through a door) to a handheld variant. Another company working on the same sort of thing is HSV Technologies.
The voltage requirements to replicate lightning with this system are well known, and are currently around $10^8$ to $10^9$ volts. Right now, this is a tremendous amount of power, but can be obtained by using step-up transformers to trade current for voltage. (Ideal 'stun gun' uses high voltage low amperage anyway) Seeing as lightning has a typical voltage of only $10^8$ volts, this is to be expected. However, you can dial back to only ten to twenty thousand volts if you are aiming it at a person with intent to stun, not kill. That's easy, we put that in hand-held stun guns all the time.
The challenge remaining is not so much 'can we produce a strong enough voltage to drop someone.' Plasma is a reasonably good conductor with very low resistance. Much of the energy released into the beam will strike the target. The challenge is 'can we power a strong enough laser to create the plasma channel.' Again, the answer is 'yes.' The power of the laser would dictate the range it could fire before completely falling apart, but you could throw a strong enough laser for a 'pistol-range' shot pretty easily. You aren't trying to burn anyone with the laser, just ionize the air. It doesn't take too much to do that.
To address a few of your other questions.
There would be a small amount of scattering, but electricity prefers to follow the path of least resistance...and plasma is an awesome conductor.
There is no such thing as 'positive' or 'negative' electricity. Electricity which is the movement of electrons, which only carry a negative charge. If you want something that behaves like electricity but has a positive charge, you need positrons. But then you are playing with antimatter, thus boom. If you want to vary the lethality of the bolt, adjust the voltage or amperage (an amperage between 100 and 200 mA will stop your heart). It's the same way you can taze someone to disable them, or electric-chair someone to kill them.
A few notes of caution...
Be careful using such a weapon during a storm, as the Plasma Channel created is very friendly to natural lightning, and may invite a bolt to jump down into the channel.
Be careful using this weapon near high voltage power lines, the presence of such a strong conductor may cause power to arc out of the lines and jump into the plasma channel.
Ensure the weapon is well insulated so that it cannot ground itself out through you, and must ground itself through the target.
Ensure proper timing of the laser/electrical pulse, otherwise the beam may not have sufficiently ionized the air, and the surge of electricity will behave according to physics (and might jump back at the gunner).
All that taken into consideration, here are a few other things an Electrolaser is capable of.
Burning out electronics, including the ignition and alternator systems of an automobile. To control natural lightning strikes: there are designs out there for firing a high power laser through a thunderhead to 'aim' natural lightning strikes at targets.