Just my humble two cents worth here, but there is some passage somewhere where Buddha says 'volition is kamma', from which I infer that it is the intention which is the root of the kammic result, as expanded upon in Dhammapada 1.1:
Mind precedes all mental states. Mind is their chief; they are all mind-wrought. If with an impure mind a person speaks or acts suffering follows him like the wheel that follows the foot of the ox.
I infer from this that if one has the intent of drowning someone, and then carries it out, that unwholesome mind-state acted upon has a comparable kammic result ("...If with an impure mind a person speaks or acts..."). But if the state of mind is only of carelessness, whether a prank or ordinary ill-will but without the intent to kill, the corresponding kammic result would not be the same as if the intent had been to kill (though I can't imagine it would be a very good result). Recklessness is, of course, also unwholesome, and can have sad consequences, as I'm sure lots of us here would know from our own lives. But I recall the Buddha saying that kammas and their particular outcomes are inscrutable to ordinary beings, and that only (a Buddha?) can comprehend them (is that correct?).
NB: If I have explained any detail incorrectly, please may someone with more realization and / or erudition correct me. I would not wish to misrepresent the Teaching in any way.
Then the Blessed One, picking up a tiny bit of dust with the tip of his fingernail, said to the monk, "There isn't even this much form...feeling...
perception...fabrications...consciousness that is constant, lasting, eternal, not subject to change, that will stay just as it is as long as eternity."