I've Had So Much I Might End This Sentence in Mid

Okay, hypothetically, if you're going on a mission to, say, catch a guilty whale. And while you were catching the whale, you saw something else that may be another whale, and you were like, "What?" But then you thought, "Maybe it's not a whale. Maybe it's a big fish. Maybe it's a submarine with a face painted on it." The point is, if I kill the first whale, am I technically a murderer?