Does Veigar's Baleful Strike (Q) need to land the killing blow on a champion to gain AP or is it passive?

The tool tip is very vague in its description. I know you need to kill a minion with Q to gain AP, but how does it interact with champions?

BaleFul Strike:

(Passive): Veigar gains bonus ability power for each champion kill from any damage source. Ability Power Gain per Champion Kill: 1 / 2 / 3 / 4 / 5

(Active): Unleashes dark energy at target enemy, dealing magic damage. If this ability deals a killing blow, Veigar gains 1 ability power.

You only need to kill the enemy champion to get the passive AP increase. You will however also get an additional AP increase from last hitting that champion using Baleful Strike.

Source: http://leagueoflegends.wikia.com/wiki/Veigar_the_Tiny_Master_of_Evil

Against champs, you do not need to last hit with Baleful Strike for an AP bonus. It automatically gives you a set amount equal to Baleful Strike's level. If you last hit with it, you also gain the one from the active.