Why is lightening jagged?

Answer 1

The way lightning forms isn't quite as simple as a 'giant spark' between the earth and the clouds. What happens immediately prior to the flash of lightning is that a path is created along which the lightning travels. This path, called a stepped leader, forms in a series of jagged steps each in the region of 100 metres long, each 'step' can be at a different angle and when the path is complete the main stroke of lightning is discharged along the length of these jagged steps.

Because these steps go off at different angles the path the lighgtning takes is rarely straight and it can lead to lightning branching off in different directions.

Answer 2

I think what you are trying to ask is why does lightning appear to go in strange paths.

Basically, lightning tries to find the "path of least resistance". As the link says, a channel of ions forms and the atmosphere will discharge the electricity in the thundercloud through this channel. It forms in small segments (this takes nanoseconds so you don't actually see it forming unless you have a really fast camera speed). Once the channel is established (the channel is what makes it jagged) the lightning strike will follow that channel (sometimes numerous times).