The sculk charge can not turn blocks into Sculk Blocks, without going through the process of adding Sculk Vein to it. The sculk charge does not strictly move in a direction, instead it can loop back and forward before continuing going through a straight line of Sculk Blocks. The sculk charge does not always pick the same route, regardless if there's a Sculk Vein on a block.

The sculk charge will only generate a Sculk Sensor or Sculk Shrieker if it is at least 4 blocks away from the Sculk Catalyst that created the charge, not to be confused with death location.

If a single standing Sculk Block containing a sculk charge is destroyed, the sculk charge will vanish. The sculk charge will not remain forever, regardless wether it is currently spreading or not. However, if a Sculk Block is around, it will accept the charge. Spawning Zombie from a spawn egg into Lava located 1 block underneath close to the catalyst will not cause any spread. There are certain conditions required for the Sculk Catalyst to spread towards other blocks. Sculk blocks allow the charge created by sculk catalysts to travel through them, allowing the charge to convert blocks further away. Instructions: more information on how exactly sculk charge spreading works