It's the specific energy pattern of a biological being I just refer to it as a soul because it's easier then naming it something else, also I can easily change the speed at which the orbs fly. However, know that Grimm isn't going to be going out starting fights, that isn't a part of his character. The worst he's gonna do is make someone else fall on top of another to get some giggles. The exploding orbs are to defend himself by breaking a couple bones to stop his attacker, also he can absorb the souls without the need to kill someone else, he can just follow a mass murdering psychopath to get what he wants. Or go to a hospital, and wait for the sick to die, heck I've even been thinking of making his alter ego a doctor.